* chore: add tests for custom extension
* feat: cusom source extension #286
* fix: path to test directory
* add getter + change setter name for file extension
* add tests of source file extension validation
* fix: add validation for file extension names
* fix: property importExt -> importFileExt
* fix: redundant check (no resetting)
* fix: failing test wich did not follow the new spec
* chore: add detailed description of the test
* chore: fix doc comment to be descriptive
* docs: add note about customizing the file extension
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