Improve the detection of mp4 types #86

[ivangolubykh]

Fixes dart-lang/tools#419

I added MPEG-4 types based on the article https://www.file-recovery.com/mp4-signature-format.htm
avc1 = 61 76 63 31
iso2 = 69 73 6F 32
isom = 69 73 6F 6D
mmp4 = 6D 6D 70 34
mp41 = 6D 70 34 31
mp42 = 6D 70 34 32
mp71 = 6D 70 37 31
msnv = 6D 73 6E 76
ndas = 6E 64 61 73
ndsc = 6E 64 73 63
ndsh = 6E 64 73 68
ndsm = 6E 64 73 6D
ndsp = 6E 64 73 70
ndss = 6E 64 73 73
ndxc = 6E 64 78 63
ndxh = 6E 64 78 68
ndxm = 6E 64 78 6D
ndxp = 6E 64 78 70
ndxs = 6E 64 78 73

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• [ V ] I’ve reviewed the contributor guide and applied the relevant portions to this PR.

[natebosch]

This looks like it removes the version with the last 4 bytes [0x33, 0x67, 0x70, 0x35] or 3gp5

According to https://www.ftyps.com this mime type should be video/3gpp though - so I don't know if it was useful when it was mapped to video/mp4. I think it's OK to overwrite this old entry with the new one.

Looking at just the types that are marked as registered and map to video/mp4 it looks fairly close to your list. However there are some differences.

• mp71 does not link directly to the mime type video/mp4, this resource lists it as "various" - maybe it's OK to include anyway?
• msnv is capitalized as MSNV and maps to audio/mp4
• ndsc through ndxs are all capitalized, and none are marked as registered.

I think we can probably prune the list down to avc1, iso2, isom, mp41, and mp42

[ivangolubykh]

I returned the type "3gp5 = 33 67 70 35" and marked it as video/3gpp. And also I pruned the MP4 list down to avc1, iso2, isom, mp41 and mp42.

[natebosch]

I'm not sure if the ISO standard would have an even more authoritative list than https://www.ftyps.com/

https://www.iso.org/standard/83102.html

I don't think it's worth buying it to check though... I think it's probably safe to go with the pruned list I mentioned above.