mp4 media file opens as text document

I've downloaded a video which is being shown as a document file in finder. When clicked on, it opens with TextEdit instead of QuickTime player. When I run the following command in terminal,

vid

the output is "vid: ISO Media, MP4 Base Media v1 [IS0 14496-12:2003]." I have other mp4 videos which give the same output as above but open normally with QuickTime Player. How do I convert the document file to a playable mp4 file?