add Content-Disposition and Filename to the payload of incoming webhooks

for each message part. The ContentDisposition value is the base value without
header key/value parameters. the Filename field is the likely filename of the
part. the different email clients encode filenames differently. there is a
standard mime mechanism from rfc 2231. and there is the q/b-word-encoding from
rfc 2047. instead of letting users of the webhook api deal with those
differences, we provide just the parsed filename.

for issue #258 by morki, thanks for reporting!

message/part.go

@@ -598,6 +598,70 @@ func (p *Part) IsDSN() bool {
 		(p.Parts[1].MediaSubType == "DELIVERY-STATUS" || p.Parts[1].MediaSubType == "GLOBAL-DELIVERY-STATUS")
 }
 
+var ErrParamEncoding = errors.New("bad header parameter encoding")
+
+// DispositionFilename tries to parse the disposition header and the "filename"
+// parameter. If the filename parameter is absent or can't be parsed, the "name"
+// parameter from the Content-Type header is used for the filename. The returned
+// filename is decoded according to RFC 2231 or RFC 2047. This is a best-effort
+// attempt to find a filename for a part. If no Content-Disposition header, or
+// filename was found, empty values without error are returned.
+//
+// If the returned error is an ErrParamEncoding, it can be treated as a diagnostic
+// and a filename may still be returned.
+func (p *Part) DispositionFilename() (disposition string, filename string, err error) {
+	h, err := p.Header()
+	if err != nil {
+		return "", "", fmt.Errorf("parsing header: %v", err)
+	}
+	var disp string
+	var params map[string]string
+	cd := h.Get("Content-Disposition")
+	if cd != "" {
+		disp, params, err = mime.ParseMediaType(cd)
+	}
+	if err != nil {
+		return "", "", fmt.Errorf("%w: parsing disposition header: %v", ErrParamEncoding, err)
+	}
+	filename, err = tryDecodeParam(params["filename"])
+	if filename == "" {
+		s, err2 := tryDecodeParam(p.ContentTypeParams["name"])
+		filename = s
+		if err == nil {
+			err = err2
+		}
+	}
+	return disp, filename, err
+}
+
+// Attempt q/b-word-decode name, coming from Content-Type "name" field or
+// Content-Disposition "filename" field.
+//
+// RFC 2231 specifies an encoding for non-ascii values in mime header parameters. But
+// it appears common practice to instead just q/b-word encode the values.
+// Thunderbird and gmail.com do this for the Content-Type "name" parameter.
+// gmail.com also does that for the Content-Disposition "filename" parameter, where
+// Thunderbird uses the RFC 2231-defined encoding. Go's mime.ParseMediaType parses
+// the mechanism specified in RFC 2231 only. The value for "name" we get here would
+// already be decoded properly for standards-compliant headers, like
+// "filename*0*=UTF-8”%...; filename*1*=%.... We'll look for Q/B-word encoding
+// markers ("=?"-prefix or "?="-suffix) and try to decode if present. This would
+// only cause trouble for filenames having this prefix/suffix.
+func tryDecodeParam(name string) (string, error) {
+	if name == "" || !strings.HasPrefix(name, "=?") && !strings.HasSuffix(name, "?=") {
+		return name, nil
+	}
+	// todo: find where this is allowed. it seems quite common. perhaps we should remove the pedantic check?
+	if Pedantic {
+		return name, fmt.Errorf("%w: attachment contains rfc2047 q/b-word-encoded mime parameter instead of rfc2231-encoded", ErrParamEncoding)
+	}
+	s, err := wordDecoder.DecodeHeader(name)
+	if err != nil {
+		return name, fmt.Errorf("%w: q/b-word decoding mime parameter: %v", ErrParamEncoding, err)
+	}
+	return s, nil
+}
+
 // Reader returns a reader for the decoded body content.
 func (p *Part) Reader() io.Reader {
 	return p.bodyReader(p.RawReader())

queue/hook.go

@@ -796,13 +796,18 @@ func Incoming(ctx context.Context, log mlog.Log, acc *store.Account, messageID s
 
 		log.Debug("composing webhook for incoming message")
 
+		structure, err := webhook.PartStructure(log, &part)
+		if err != nil {
+			return fmt.Errorf("parsing part structure: %v", err)
+		}
+
 		isIncoming = true
 		var rcptTo string
 		if m.RcptToDomain != "" {
 			rcptTo = m.RcptToLocalpart.String() + "@" + m.RcptToDomain
 		}
 		in := webhook.Incoming{
-			Structure: webhook.PartStructure(&part),
+			Structure: structure,
 			Meta: webhook.IncomingMeta{
 				MsgID:               m.ID,
 				MailFrom:            m.MailFrom,

queue/hook_test.go

@@ -82,6 +82,9 @@ func TestHookIncoming(t *testing.T) {
 		tcheck(t, err, "decode incoming webhook")
 		in.Meta.Received = in.Meta.Received.Local() // For TZ UTC.
 
+		structure, err := webhook.PartStructure(pkglog, &part)
+		tcheck(t, err, "part structure")
+
 		expIncoming := webhook.Incoming{
 			From:       []webhook.NameAddress{{Address: "mjl@mox.example"}},
 			To:         []webhook.NameAddress{{Address: "mjl@mox.example"}},
@@ -92,7 +95,7 @@ func TestHookIncoming(t *testing.T) {
 			Subject:    "test",
 			Text:       "test email\n",
 
-			Structure: webhook.PartStructure(&part),
+			Structure: structure,
 			Meta: webhook.IncomingMeta{
 				MsgID:               m.ID,
 				MailFrom:            m.MailFrom,

webaccount/account.js

@@ -278,7 +278,7 @@ var api;
 		"Outgoing": { "Name": "Outgoing", "Docs": "", "Fields": [{ "Name": "Version", "Docs": "", "Typewords": ["int32"] }, { "Name": "Event", "Docs": "", "Typewords": ["OutgoingEvent"] }, { "Name": "DSN", "Docs": "", "Typewords": ["bool"] }, { "Name": "Suppressing", "Docs": "", "Typewords": ["bool"] }, { "Name": "QueueMsgID", "Docs": "", "Typewords": ["int64"] }, { "Name": "FromID", "Docs": "", "Typewords": ["string"] }, { "Name": "MessageID", "Docs": "", "Typewords": ["string"] }, { "Name": "Subject", "Docs": "", "Typewords": ["string"] }, { "Name": "WebhookQueued", "Docs": "", "Typewords": ["timestamp"] }, { "Name": "SMTPCode", "Docs": "", "Typewords": ["int32"] }, { "Name": "SMTPEnhancedCode", "Docs": "", "Typewords": ["string"] }, { "Name": "Error", "Docs": "", "Typewords": ["string"] }, { "Name": "Extra", "Docs": "", "Typewords": ["{}", "string"] }] },
 		"Incoming": { "Name": "Incoming", "Docs": "", "Fields": [{ "Name": "Version", "Docs": "", "Typewords": ["int32"] }, { "Name": "From", "Docs": "", "Typewords": ["[]", "NameAddress"] }, { "Name": "To", "Docs": "", "Typewords": ["[]", "NameAddress"] }, { "Name": "CC", "Docs": "", "Typewords": ["[]", "NameAddress"] }, { "Name": "BCC", "Docs": "", "Typewords": ["[]", "NameAddress"] }, { "Name": "ReplyTo", "Docs": "", "Typewords": ["[]", "NameAddress"] }, { "Name": "Subject", "Docs": "", "Typewords": ["string"] }, { "Name": "MessageID", "Docs": "", "Typewords": ["string"] }, { "Name": "InReplyTo", "Docs": "", "Typewords": ["string"] }, { "Name": "References", "Docs": "", "Typewords": ["[]", "string"] }, { "Name": "Date", "Docs": "", "Typewords": ["nullable", "timestamp"] }, { "Name": "Text", "Docs": "", "Typewords": ["string"] }, { "Name": "HTML", "Docs": "", "Typewords": ["string"] }, { "Name": "Structure", "Docs": "", "Typewords": ["Structure"] }, { "Name": "Meta", "Docs": "", "Typewords": ["IncomingMeta"] }] },
 		"NameAddress": { "Name": "NameAddress", "Docs": "", "Fields": [{ "Name": "Name", "Docs": "", "Typewords": ["string"] }, { "Name": "Address", "Docs": "", "Typewords": ["string"] }] },
-		"Structure": { "Name": "Structure", "Docs": "", "Fields": [{ "Name": "ContentType", "Docs": "", "Typewords": ["string"] }, { "Name": "ContentTypeParams", "Docs": "", "Typewords": ["{}", "string"] }, { "Name": "ContentID", "Docs": "", "Typewords": ["string"] }, { "Name": "DecodedSize", "Docs": "", "Typewords": ["int64"] }, { "Name": "Parts", "Docs": "", "Typewords": ["[]", "Structure"] }] },
+		"Structure": { "Name": "Structure", "Docs": "", "Fields": [{ "Name": "ContentType", "Docs": "", "Typewords": ["string"] }, { "Name": "ContentTypeParams", "Docs": "", "Typewords": ["{}", "string"] }, { "Name": "ContentID", "Docs": "", "Typewords": ["string"] }, { "Name": "ContentDisposition", "Docs": "", "Typewords": ["string"] }, { "Name": "Filename", "Docs": "", "Typewords": ["string"] }, { "Name": "DecodedSize", "Docs": "", "Typewords": ["int64"] }, { "Name": "Parts", "Docs": "", "Typewords": ["[]", "Structure"] }] },
 		"IncomingMeta": { "Name": "IncomingMeta", "Docs": "", "Fields": [{ "Name": "MsgID", "Docs": "", "Typewords": ["int64"] }, { "Name": "MailFrom", "Docs": "", "Typewords": ["string"] }, { "Name": "MailFromValidated", "Docs": "", "Typewords": ["bool"] }, { "Name": "MsgFromValidated", "Docs": "", "Typewords": ["bool"] }, { "Name": "RcptTo", "Docs": "", "Typewords": ["string"] }, { "Name": "DKIMVerifiedDomains", "Docs": "", "Typewords": ["[]", "string"] }, { "Name": "RemoteIP", "Docs": "", "Typewords": ["string"] }, { "Name": "Received", "Docs": "", "Typewords": ["timestamp"] }, { "Name": "MailboxName", "Docs": "", "Typewords": ["string"] }, { "Name": "Automated", "Docs": "", "Typewords": ["bool"] }] },
 		"TLSPublicKey": { "Name": "TLSPublicKey", "Docs": "", "Fields": [{ "Name": "Fingerprint", "Docs": "", "Typewords": ["string"] }, { "Name": "Created", "Docs": "", "Typewords": ["timestamp"] }, { "Name": "Type", "Docs": "", "Typewords": ["string"] }, { "Name": "Name", "Docs": "", "Typewords": ["string"] }, { "Name": "NoIMAPPreauth", "Docs": "", "Typewords": ["bool"] }, { "Name": "CertDER", "Docs": "", "Typewords": ["nullable", "string"] }, { "Name": "Account", "Docs": "", "Typewords": ["string"] }, { "Name": "LoginAddress", "Docs": "", "Typewords": ["string"] }] },
 		"CSRFToken": { "Name": "CSRFToken", "Docs": "", "Values": null },
@@ -1390,6 +1390,8 @@ const index = async () => {
 				ContentType: 'text/plain',
 				ContentTypeParams: { charset: 'utf-8' },
 				ContentID: '',
+				ContentDisposition: '',
+				Filename: '',
 				DecodedSize: 8,
 				Parts: [],
 			},

webaccount/account.ts

@@ -669,6 +669,8 @@ const index = async () => {
 				ContentType: 'text/plain',
 				ContentTypeParams: {charset: 'utf-8'},
 				ContentID: '',
+				ContentDisposition: '',
+				Filename: '',
 				DecodedSize: 8,
 				Parts: [],
 			},

webaccount/api.json

@@ -1495,6 +1495,20 @@
 						"string"
 					]
 				},
+				{
+					"Name": "ContentDisposition",
+					"Docs": "Lower-case value, e.g. \"attachment\", \"inline\" or empty when absent. Without the key/value header parameters.",
+					"Typewords": [
+						"string"
+					]
+				},
+				{
+					"Name": "Filename",
+					"Docs": "Filename for this part, based on \"filename\" parameter from Content-Disposition, or \"name\" from Content-Type after decoding.",
+					"Typewords": [
+						"string"
+					]
+				},
 				{
 					"Name": "DecodedSize",
 					"Docs": "Size of content after decoding content-transfer-encoding. For text and HTML parts, this can be larger than the data returned since this size includes \\r\\n line endings.",

webaccount/api.ts

@@ -187,6 +187,8 @@ export interface Structure {
 	ContentType: string  // Lower case, e.g. text/plain.
 	ContentTypeParams?: { [key: string]: string }  // Lower case keys, original case values, e.g. {"charset": "UTF-8"}.
 	ContentID: string  // Can be empty. Otherwise, should be a value wrapped in <>'s. For use in HTML, referenced as URI `cid:...`.
+	ContentDisposition: string  // Lower-case value, e.g. "attachment", "inline" or empty when absent. Without the key/value header parameters.
+	Filename: string  // Filename for this part, based on "filename" parameter from Content-Disposition, or "name" from Content-Type after decoding.
 	DecodedSize: number  // Size of content after decoding content-transfer-encoding. For text and HTML parts, this can be larger than the data returned since this size includes \r\n line endings.
 	Parts?: Structure[] | null  // Subparts of a multipart message, possibly recursive.
 }
@@ -274,7 +276,7 @@ export const types: TypenameMap = {
 	"Outgoing": {"Name":"Outgoing","Docs":"","Fields":[{"Name":"Version","Docs":"","Typewords":["int32"]},{"Name":"Event","Docs":"","Typewords":["OutgoingEvent"]},{"Name":"DSN","Docs":"","Typewords":["bool"]},{"Name":"Suppressing","Docs":"","Typewords":["bool"]},{"Name":"QueueMsgID","Docs":"","Typewords":["int64"]},{"Name":"FromID","Docs":"","Typewords":["string"]},{"Name":"MessageID","Docs":"","Typewords":["string"]},{"Name":"Subject","Docs":"","Typewords":["string"]},{"Name":"WebhookQueued","Docs":"","Typewords":["timestamp"]},{"Name":"SMTPCode","Docs":"","Typewords":["int32"]},{"Name":"SMTPEnhancedCode","Docs":"","Typewords":["string"]},{"Name":"Error","Docs":"","Typewords":["string"]},{"Name":"Extra","Docs":"","Typewords":["{}","string"]}]},
 	"Incoming": {"Name":"Incoming","Docs":"","Fields":[{"Name":"Version","Docs":"","Typewords":["int32"]},{"Name":"From","Docs":"","Typewords":["[]","NameAddress"]},{"Name":"To","Docs":"","Typewords":["[]","NameAddress"]},{"Name":"CC","Docs":"","Typewords":["[]","NameAddress"]},{"Name":"BCC","Docs":"","Typewords":["[]","NameAddress"]},{"Name":"ReplyTo","Docs":"","Typewords":["[]","NameAddress"]},{"Name":"Subject","Docs":"","Typewords":["string"]},{"Name":"MessageID","Docs":"","Typewords":["string"]},{"Name":"InReplyTo","Docs":"","Typewords":["string"]},{"Name":"References","Docs":"","Typewords":["[]","string"]},{"Name":"Date","Docs":"","Typewords":["nullable","timestamp"]},{"Name":"Text","Docs":"","Typewords":["string"]},{"Name":"HTML","Docs":"","Typewords":["string"]},{"Name":"Structure","Docs":"","Typewords":["Structure"]},{"Name":"Meta","Docs":"","Typewords":["IncomingMeta"]}]},
 	"NameAddress": {"Name":"NameAddress","Docs":"","Fields":[{"Name":"Name","Docs":"","Typewords":["string"]},{"Name":"Address","Docs":"","Typewords":["string"]}]},
-	"Structure": {"Name":"Structure","Docs":"","Fields":[{"Name":"ContentType","Docs":"","Typewords":["string"]},{"Name":"ContentTypeParams","Docs":"","Typewords":["{}","string"]},{"Name":"ContentID","Docs":"","Typewords":["string"]},{"Name":"DecodedSize","Docs":"","Typewords":["int64"]},{"Name":"Parts","Docs":"","Typewords":["[]","Structure"]}]},
+	"Structure": {"Name":"Structure","Docs":"","Fields":[{"Name":"ContentType","Docs":"","Typewords":["string"]},{"Name":"ContentTypeParams","Docs":"","Typewords":["{}","string"]},{"Name":"ContentID","Docs":"","Typewords":["string"]},{"Name":"ContentDisposition","Docs":"","Typewords":["string"]},{"Name":"Filename","Docs":"","Typewords":["string"]},{"Name":"DecodedSize","Docs":"","Typewords":["int64"]},{"Name":"Parts","Docs":"","Typewords":["[]","Structure"]}]},
 	"IncomingMeta": {"Name":"IncomingMeta","Docs":"","Fields":[{"Name":"MsgID","Docs":"","Typewords":["int64"]},{"Name":"MailFrom","Docs":"","Typewords":["string"]},{"Name":"MailFromValidated","Docs":"","Typewords":["bool"]},{"Name":"MsgFromValidated","Docs":"","Typewords":["bool"]},{"Name":"RcptTo","Docs":"","Typewords":["string"]},{"Name":"DKIMVerifiedDomains","Docs":"","Typewords":["[]","string"]},{"Name":"RemoteIP","Docs":"","Typewords":["string"]},{"Name":"Received","Docs":"","Typewords":["timestamp"]},{"Name":"MailboxName","Docs":"","Typewords":["string"]},{"Name":"Automated","Docs":"","Typewords":["bool"]}]},
 	"TLSPublicKey": {"Name":"TLSPublicKey","Docs":"","Fields":[{"Name":"Fingerprint","Docs":"","Typewords":["string"]},{"Name":"Created","Docs":"","Typewords":["timestamp"]},{"Name":"Type","Docs":"","Typewords":["string"]},{"Name":"Name","Docs":"","Typewords":["string"]},{"Name":"NoIMAPPreauth","Docs":"","Typewords":["bool"]},{"Name":"CertDER","Docs":"","Typewords":["nullable","string"]},{"Name":"Account","Docs":"","Typewords":["string"]},{"Name":"LoginAddress","Docs":"","Typewords":["string"]}]},
 	"CSRFToken": {"Name":"CSRFToken","Docs":"","Values":null},

webapi/doc.go

@@ -347,6 +347,8 @@ Example JSON body for webhooks for incoming delivery of basic message:
 				"charset": "utf-8"
 			},
 			"ContentID": "",
+			"ContentDisposition": "",
+			"Filename": "",
 			"DecodedSize": 17,
 			"Parts": []
 		},

webapisrv/server.go

@@ -1263,9 +1263,12 @@ func (s server) MessageGet(ctx context.Context, req webapi.MessageGetRequest) (r
 		MailboxName:         mb.Name,
 	}
 
+	structure, err := webhook.PartStructure(log, &p)
+	xcheckf(err, "parsing structure")
+
 	result := webapi.MessageGetResult{
 		Message:   msg,
-		Structure: webhook.PartStructure(&p),
+		Structure: structure,
 		Meta:      meta,
 	}
 	return result, nil

webapisrv/server_test.go

@@ -418,7 +418,9 @@ func TestServer(t *testing.T) {
 	tcheckf(t, err, "reading raw message")
 	part, err := message.EnsurePart(log.Logger, true, bytes.NewReader(b.Bytes()), int64(b.Len()))
 	tcheckf(t, err, "parsing raw message")
-	tcompare(t, webhook.PartStructure(&part), msgRes.Structure)
+	structure, err := webhook.PartStructure(log, &part)
+	tcheckf(t, err, "part structure")
+	tcompare(t, structure, msgRes.Structure)
 
 	_, err = client.MessageRawGet(ctxbg, webapi.MessageRawGetRequest{MsgID: 1 + 999})
 	terrcode(t, err, "messageNotFound")

webhook/webhook.go

@@ -8,10 +8,12 @@
 package webhook
 
 import (
+	"errors"
 	"strings"
 	"time"
 
 	"github.com/mjl-/mox/message"
+	"github.com/mjl-/mox/mlog"
 )
 
 // OutgoingEvent is an activity for an outgoing delivery. Either generated by the
@@ -138,20 +140,34 @@ type Structure struct {
 	ContentType        string            // Lower case, e.g. text/plain.
 	ContentTypeParams  map[string]string // Lower case keys, original case values, e.g. {"charset": "UTF-8"}.
 	ContentID          string            // Can be empty. Otherwise, should be a value wrapped in <>'s. For use in HTML, referenced as URI `cid:...`.
+	ContentDisposition string            // Lower-case value, e.g. "attachment", "inline" or empty when absent. Without the key/value header parameters.
+	Filename           string            // Filename for this part, based on "filename" parameter from Content-Disposition, or "name" from Content-Type after decoding.
 	DecodedSize        int64             // Size of content after decoding content-transfer-encoding. For text and HTML parts, this can be larger than the data returned since this size includes \r\n line endings.
 	Parts              []Structure       // Subparts of a multipart message, possibly recursive.
 }
 
 // PartStructure returns a Structure for a parsed message part.
-func PartStructure(p *message.Part) Structure {
+func PartStructure(log mlog.Log, p *message.Part) (Structure, error) {
 	parts := make([]Structure, len(p.Parts))
 	for i := range p.Parts {
-		parts[i] = PartStructure(&p.Parts[i])
+		var err error
+		parts[i], err = PartStructure(log, &p.Parts[i])
+		if err != nil && !errors.Is(err, message.ErrParamEncoding) {
+			return Structure{}, err
+		}
+	}
+	disp, filename, err := p.DispositionFilename()
+	if err != nil && errors.Is(err, message.ErrParamEncoding) {
+		log.Debugx("parsing disposition/filename", err)
+	} else if err != nil {
+		return Structure{}, err
 	}
 	s := Structure{
 		ContentType:        strings.ToLower(p.MediaType + "/" + p.MediaSubType),
 		ContentTypeParams:  p.ContentTypeParams,
 		ContentID:          p.ContentID,
+		ContentDisposition: strings.ToLower(disp),
+		Filename:           filename,
 		DecodedSize:        p.DecodedSize,
 		Parts:              parts,
 	}
@@ -159,5 +175,5 @@ func PartStructure(p *message.Part) Structure {
 	if s.ContentTypeParams == nil {
 		s.ContentTypeParams = map[string]string{}
 	}
-	return s
+	return s, nil
 }

webmail/api.go

@@ -904,7 +904,12 @@ func (w Webmail) MessageSubmit(ctx context.Context, m SubmitMessage) {
 							ap = ap.Parts[xp]
 						}
 
-						filename := tryDecodeParam(log, ap.ContentTypeParams["name"])
+						_, filename, err := ap.DispositionFilename()
+						if err != nil && errors.Is(err, message.ErrParamEncoding) {
+							log.Debugx("parsing disposition/filename", err)
+						} else {
+							xcheckf(ctx, err, "reading disposition")
+						}
 						if filename == "" {
 							filename = "unnamed.bin"
 						}

webmail/message.go

@@ -2,6 +2,7 @@ package webmail
 
 import (
 	"bufio"
+	"errors"
 	"fmt"
 	"io"
 	"log/slog"
@@ -275,21 +276,16 @@ func parsedMessage(log mlog.Log, m store.Message, state *msgState, full, msgitem
 		case "TEXT/PLAIN", "/":
 			// Don't include if Content-Disposition attachment.
 			if full || msgitem {
-				// todo: should have this, and perhaps all content-* headers, preparsed in message.Part?
-				h, err := p.Header()
-				log.Check(err, "parsing attachment headers", slog.Int64("msgid", m.ID))
-				cp := h.Get("Content-Disposition")
-				if cp != "" {
-					disp, params, err := mime.ParseMediaType(cp)
-					log.Check(err, "parsing content-disposition", slog.String("cp", cp))
-					if strings.EqualFold(disp, "attachment") {
-						name := tryDecodeParam(log, p.ContentTypeParams["name"])
-						if name == "" {
-							name = tryDecodeParam(log, params["filename"])
-						}
-						addAttachment(Attachment{path, name, p}, parentMixed)
+				disp, name, err := p.DispositionFilename()
+				if err != nil && errors.Is(err, message.ErrParamEncoding) {
+					log.Debugx("parsing disposition/filename", err)
+				} else if err != nil {
+					rerr = fmt.Errorf("reading disposition/filename: %v", err)
 					return
 				}
+				if strings.EqualFold(disp, "attachment") {
+					addAttachment(Attachment{path, name, p}, parentMixed)
+					return
 				}
 			}
 

webmail/view.go

@@ -1829,11 +1829,17 @@ func attachmentTypes(log mlog.Log, m store.Message, state *msgState) (map[Attach
 		mt := strings.ToLower(a.Part.MediaType + "/" + a.Part.MediaSubType)
 		if t, ok := attachmentMimetypes[mt]; ok {
 			types[t] = true
-		} else if ext := filepath.Ext(tryDecodeParam(log, a.Part.ContentTypeParams["name"])); ext != "" {
+			continue
+		}
+		_, filename, err := a.Part.DispositionFilename()
+		if err != nil && errors.Is(err, message.ErrParamEncoding) {
+			log.Debugx("parsing disposition/filename", err)
+		} else if err != nil {
+			return nil, fmt.Errorf("reading disposition/filename: %v", err)
+		}
+		if ext := filepath.Ext(filename); ext != "" {
 			if t, ok := attachmentExtensions[strings.ToLower(ext)]; ok {
 				types[t] = true
-			} else {
-				continue
 			}
 		}
 	}

webmail/webmail.go

@@ -521,13 +521,11 @@ func handle(apiHandler http.Handler, isForwarded bool, accountPath string, w htt
 		names := map[string]bool{}
 		for _, a := range mi.Attachments {
 			ap := a.Part
-			name := tryDecodeParam(log, ap.ContentTypeParams["name"])
-			if name == "" {
-				// We don't check errors, this is all best-effort.
-				h, _ := ap.Header()
-				disposition := h.Get("Content-Disposition")
-				_, params, _ := mime.ParseMediaType(disposition)
-				name = tryDecodeParam(log, params["filename"])
+			_, name, err := ap.DispositionFilename()
+			if err != nil && errors.Is(err, message.ErrParamEncoding) {
+				log.Debugx("parsing disposition header for filename", err)
+			} else {
+				xcheckf(ctx, err, "reading disposition header")
 			}
 			if name != "" {
 				name = filepath.Base(name)
@@ -816,13 +814,11 @@ func handle(apiHandler http.Handler, isForwarded bool, accountPath string, w htt
 		h.Set("Content-Type", ct)
 		h.Set("Cache-Control", "no-store, max-age=0")
 		if t[1] == "download" {
-			name := tryDecodeParam(log, ap.ContentTypeParams["name"])
-			if name == "" {
-				// We don't check errors, this is all best-effort.
-				h, _ := ap.Header()
-				disposition := h.Get("Content-Disposition")
-				_, params, _ := mime.ParseMediaType(disposition)
-				name = tryDecodeParam(log, params["filename"])
+			_, name, err := ap.DispositionFilename()
+			if err != nil && errors.Is(err, message.ErrParamEncoding) {
+				log.Debugx("parsing disposition/filename", err)
+			} else {
+				xcheckf(ctx, err, "reading disposition/filename")
 			}
 			if name == "" {
 				name = "attachment.bin"