[mothered]

In addition to the above post, this site does the job well.

http://www.corruptmyfile.com/

I've downloaded and tested a random file In MP4 format (fully functional at the time), and It served It's purpose nicely.

Spoiler:

Spoiler:

Spoiler:

Take note of the error message In the last Image, this Is very Important. It provides a few reasons as to why the file Is not functioning. Furthermore, Googling the error code has Infinite results as to It's cause. This will significantly help your SE by convincing the target that It may well be an Issue on their end.