m4v file - incorrect mime type?

I'm unable to get mimedetect to allow me to upload a m4v file. First I got this error message:

The selected file filename.m4v could not be uploaded. The file contents (video/mp4) do not match its extension (m4v).

So I looked in the mime.magic file and the m4v filetype is listed as video/mp4. I changed it to match the mime type listed in the web server settings (video/x-m4v) but then I got this error message.

The selected file filename.m4v could not be uploaded. The file contents (video/x-m4v do not match its extension (m4v).

My video people insist it's not a problem with the file. Any suggestions? Are there any negative implications for disabling mime detection here?