Minor fixes

[tvolkert]

• Allow m4v files in FileType.media filter
• throw if caller tries to use allowedExtensiosn with any file type other than custom

[navaronbracke]

We have one test that is failing, because the expectation needs to be updated.

flutter_file_picker/flutter_file_picker/test/file_picker_macos_test.dart: fileTypeToFileFilter() should return the file filter (failed)
  Expected: '"avi", "flv", "mkv", "mov", "mp4", "mpeg", "webm", "wmv", "bmp", "gif", "jpeg", "jpg", "png"'
    Actual: '"avi", "flv", "m4v", "mkv", "mov", "mp4", "mpeg", "webm", "wmv", "bmp", "gif", "jpeg", "jpg", "png"'
     Which: is different.
            Expected: ...  "flv", "mkv", "mov" ...
              Actual: ...  "flv", "m4v", "mkv" ...
                                    ^
             Differ at offset [16](https://github.com/miguelpruivo/flutter_file_picker/actions/runs/7456317251/job/20290515807?pr=1430#step:8:17)

And while I'm talking about tests, could you perhaps add a test for the ArgumentError as well? (one for file_picker_io and one for file_picker_macos)

[miguelpruivo]

LGTM. Thanks.