Remove unsupported formats

[deldesir]

Stop supporting uploads for files whose formats are not supported natively by browsers.

[holta]

If an MKV (Matroska) video comes from YouTube / ETC (as seems to happen) should we partially tolerate that?

e.g. should we (will we?) store the video for possible student/user download — even though it's not playable in browser?

[holta]

Just FWIW https://support.google.com/youtube/troubleshooter/2888402 says...

Supported YouTube file formats
Note: Audio files, like MP3, WAV, or PCM files, can’t be uploaded to create a YouTube video. You can use video editing software to convert your audio file to a video. Audio files can only be uploaded as additional languages for your video.
If you're not sure which format to save your video as or are getting an "invalid file format" error message when you're uploading, make sure that you’re using one of the following formats:

.MOV
.MPEG-1
.MPEG-2
.MPEG4
.MP4
.MPG
.AVI
.WMV
.MPEGPS
.FLV
3GPP
WebM
DNxHR
ProRes
CineForm
HEVC (h265)
If you're using a file format that's not listed above, use this troubleshooter to learn how to convert your file.

For more advanced information on file formats you can read through this article on encoding settings.

Select the file format you'd like to convert
[ ] mswmm (Movie maker project file), .msdvd (DVD Maker project file), .wlmp (Movie maker project file)
[ ] .camproj (Camtasia project file)
[ ] imovieproject, .dvdproj (iDVD project file), .rcproject (iMovie project file)
[ ] .piv
[ ] Other

[holta]

This PR is part of an ongoing community assessment as to which video format/codecs will be truly/universally required in 2024+ 👍

So for now (until we know more!) these legacy formats are being removed:

• avi, m4v, mpg, mpeg, ogv
• mov, wmv, flv, 3gp, 3g2

SEE Wiki Documentation: https://github.com/iiab/calibre-web/wiki/Features-wiki#plus-videos--images