diff --git a/webapi/client.go b/webapi/client.go
index b769df6..f094544 100644
--- a/webapi/client.go
+++ b/webapi/client.go
@@ -114,10 +114,10 @@ func badResponse(hresp *http.Response) error {
 // Configure webhooks to receive updates about deliveries.
 //
 // If the request is a multipart/form-data, uploaded files with the form keys
-// "inlinefile" and/or "attachedfile" will be added to the message. If the uploaded
-// file has content-type and/or content-id headers, they will be included. If no
-// content-type is present in the request, and it can be detected, it is included
-// automatically.
+// "alternativefile", "inlinefile" and/or "attachedfile" will be added to the
+// message. If the uploaded file has content-type and/or content-id headers, they
+// will be included. If no content-type is present in the request, and it can be
+// detected, it is included automatically.
 //
 // Example call with a text and html message, with an inline and an attached image:
 //
diff --git a/webapi/webapi.go b/webapi/webapi.go
index 1e5bda6..823cc67 100644
--- a/webapi/webapi.go
+++ b/webapi/webapi.go
@@ -115,6 +115,11 @@ type SendRequest struct {
 	// Unless a User-Agent or X-Mailer header is present, a User-Agent is added.
 	Headers [][2]string
 
+	// Alternative files are added as (full) alternative representation of the text
+	// and/or html parts. Alternative files cause a part with content-type
+	// "multipart/alternative" to be added to the message. Optional.
+	AlternativeFiles []File
+
 	// Inline files are added to the message and should be displayed by mail clients as
 	// part of the message contents. Inline files cause a part with content-type
 	// "multipart/related" to be added to the message. Optional.
diff --git a/webapisrv/server.go b/webapisrv/server.go
index 7f07553..a1daebd 100644
--- a/webapisrv/server.go
+++ b/webapisrv/server.go
@@ -759,8 +759,9 @@ func (s server) Send(ctx context.Context, req webapi.SendRequest) (resp webapi.S
 		xc.Header("User-Agent", "mox/"+moxvar.Version)
 	}
 
-	// Whether we have additional separately inline/attached file(s).
+	// Whether we have additional separately alternative/inline/attached file(s).
 	mpf := reqInfo.Request.MultipartForm
+	formAlternative := mpf != nil && len(mpf.File["alternativefile"]) > 0
 	formInline := mpf != nil && len(mpf.File["inlinefile"]) > 0
 	formAttachment := mpf != nil && len(mpf.File["attachedfile"]) > 0
 
@@ -770,6 +771,7 @@ func (s server) Send(ctx context.Context, req webapi.SendRequest) (resp webapi.S
 	//     - multipart/alternative (in case we have both text and html bodies)
 	//       - text/plain (optional)
 	//       - text/html (optional)
+	//       - alternative file, ...
 	//     - inline file, ...
 	//   - attached file, ...
 
@@ -811,7 +813,7 @@ func (s server) Send(ctx context.Context, req webapi.SendRequest) (resp webapi.S
 		related = xcreateMultipart("related")
 		cur = related
 	}
-	if m.Text != "" && m.HTML != "" {
+	if m.Text != "" && m.HTML != "" || len(req.AlternativeFiles) > 0 || formAlternative {
 		alternative = xcreateMultipart("alternative")
 		cur = alternative
 	}
@@ -827,10 +829,6 @@ func (s server) Send(ctx context.Context, req webapi.SendRequest) (resp webapi.S
 		_, err := tp.Write([]byte(htmlBody))
 		xcheckf(err, "write html part")
 	}
-	if alternative != nil {
-		alternative.Close()
-		alternative = nil
-	}
 
 	xaddFileBase64 := func(ct string, inline bool, filename string, cid string, base64Data string) {
 		h := textproto.MIMEHeader{}
@@ -923,6 +921,18 @@ func (s server) Send(ctx context.Context, req webapi.SendRequest) (resp webapi.S
 		xcheckf(err, "flushing uploaded file")
 	}
 
+	cur = alternative
+	xaddJSONFiles(req.AlternativeFiles, true)
+	if mpf != nil {
+		for _, fh := range mpf.File["alternativefile"] {
+			xaddFile(fh, true)
+		}
+	}
+	if alternative != nil {
+		alternative.Close()
+		alternative = nil
+	}
+
 	cur = related
 	xaddJSONFiles(req.InlineFiles, true)
 	if mpf != nil {
diff --git a/webapisrv/server_test.go b/webapisrv/server_test.go
index faaff75..3271098 100644
--- a/webapisrv/server_test.go
+++ b/webapisrv/server_test.go
@@ -181,6 +181,13 @@ func TestServer(t *testing.T) {
 		},
 		Extra:   map[string]string{"a": "123"},
 		Headers: [][2]string{{"x-custom", "header"}},
+		AlternativeFiles: []webapi.File{
+			{
+				Name:        "x.ics",
+				ContentType: "text/calendar",
+				Data:        base64.StdEncoding.EncodeToString([]byte("ics data...")),
+			},
+		},
 		InlineFiles: []webapi.File{
 			{
 				Name:        "x.png",
@@ -228,8 +235,15 @@ func TestServer(t *testing.T) {
 	sendReqBuf, err := json.Marshal(fdSendReq)
 	tcheckf(t, err, "send request")
 	mp.WriteField("request", string(sendReqBuf))
+
+	// One alternative file.
+	pw, err := mp.CreateFormFile("alternativefile", "test.ics")
+	tcheckf(t, err, "create alternative ics file")
+	_, err = fmt.Fprint(pw, "ICS...")
+	tcheckf(t, err, "write ics")
+
 	// Two inline PDFs.
-	pw, err := mp.CreateFormFile("inlinefile", "test.pdf")
+	pw, err = mp.CreateFormFile("inlinefile", "test.pdf")
 	tcheckf(t, err, "create inline pdf file")
 	_, err = fmt.Fprint(pw, "%PDF-")
 	tcheckf(t, err, "write pdf")