The error message you are receiving very likely indicates that you don't have a media player on your personal computer setup for .m4v type media.
I know that neither Windows Media Player or the free version of RealPlayer support that media type.
"Invalid content type returned from server" probably just means "I can't play this media type."
If the file was corrupted, you'd get a different message.
Bottom line: this is a problem with your personal computer, not your web server, WordPress, Powerpress or anything else. But, it should also alert you to the fact that this media type is not something other people are likely to be able to play either.