Error messages and what they mean
Uploaded file is not a video, or has invalid MIME type
This means the file you've uploaded isn't actually a video file. Common reasons for this would include:-
a) Video file is corrupt
b) Video is inside a compressed file (eg a .zip file)
c) Video has an invalid MIME type
Ultimately the only way to resolve this is to check the video file you've uploaded and re-upload it.