It seems like this is a recent change too since the timestamps on the uploads say that this started July 31, 2018.

Co-incided with conversion to extension registration? Possible Regression

https://commons.wikimedia.org/wiki/Special:AbuseFilter/209 as a workaround, but may be incomplete.

Change 453878 had a related patch set uploaded (by Brion VIBBER; owner: Brion VIBBER):
[mediawiki/extensions/TimedMediaHandler@master] Fix regression: double-reg of file types and incorrect 'mp4'

https://gerrit.wikimedia.org/r/453878

Yep, that's a regression in the extension.json migration. The addition of file types to $wgFileExtensions was being done twice, both in the extension.json and then again dynamically, causing the way that 'mp4' was removed from the list to fail because it only removed one instance of it.

Scheduling the fix to go out tomorrow morning pacific.

Change 453879 had a related patch set uploaded (by Brion VIBBER; owner: Brion VIBBER):
[mediawiki/extensions/TimedMediaHandler@wmf/1.32.0-wmf.16] Fix regression: double-reg of file types and incorrect 'mp4'

https://gerrit.wikimedia.org/r/453879

Change 453878 merged by jenkins-bot:
[mediawiki/extensions/TimedMediaHandler@master] Fix regression: double-reg of file types and incorrect 'mp4'

https://gerrit.wikimedia.org/r/453878

Change 453879 merged by jenkins-bot:
[mediawiki/extensions/TimedMediaHandler@wmf/1.32.0-wmf.16] Fix regression: double-reg of file types and incorrect 'mp4'

https://gerrit.wikimedia.org/r/453879

Fix has been deployed; no more MP4 files should be coming in.

Existing files should be either deleted or re-encoded.

https://en.wikipedia.org/w/index.php?title=Special:Search&limit=500&offset=0&ns6=1&search=filemime%3Amp4

Should we leave open for the community to delete?

In T202208#4515989, @Jdforrester-WMF wrote:

Should we leave open for the community to delete?

I'll do a batch run through them later today/tonight, then close out the task.

@brion Perhaps they should be left up for a while. They seem to trigger strange bugs. For example https://commons.wikimedia.org/wiki/File:Maalwerk_Volmolen_Frankenhof.mp4 which is a video that was shot in portrait mode, gets completely messed up when wiki makes vp9 out of it.

Should I file bug reports for such glitches or not as uploading mp4 should have never been possible in the first place?

Update: reported as T209560

In T202208#4517359, @AlexisJazz wrote:

@brion Perhaps they should be left up for a while. They seem to trigger strange bugs. For example https://commons.wikimedia.org/wiki/File:Maalwerk_Volmolen_Frankenhof.mp4 which is a video that was shot in portrait mode, gets completely messed up when wiki makes vp9 out of it.

Should I file bug reports for such glitches or not as uploading mp4 should have never been possible in the first place?

Yeah, file bug(s) please as appropriate.

TMH can/does/will support MP4, but just because Commons isn't allowing it, doesn't mean it's not a valid bug in one form or another, and therefore other users of TMH may benefit :)

These files are still up btw...

@TheDJ I plan to convert some more after T209557 is fixed..