I was trying to upload a MP4 video (~6 MB) to http://www.twitter.com/ After full upload got the following message
Your media file could not be processed.
I have frequently been seeing this message for all MP4 types whereas the same is acceptable by Facebook engine.
We are unable to support questions about the Twitter website via the developer forums. If you’ve tried uploading via the API and your media file matches our specifications then please provide a link to the file so someone can try to reproduce the issue.
https://developer.twitter.com/en/docs/media/upload-media/uploading-media/media-best-practices.html