Why is GET request for mp4 file not working in Safari?

I'm trying to download an mp4 file using the Javascript JWPlayer API, but when I do it in Safari, the GET request to fetch the video file fails. Looking at the GET request, I think I can understand why: the MIME type specified for the request is "undefined", whereas the type must be "video / mp4".

After a bit of research, I cannot find / understand any way to solve this problem.

I already have the mp4 file extension associated with the video / mp 4 MIME type on my server. I am using IIS.

This problem does not occur when I try to download a video in Chrome or IE. This is only a problem with Safari.

This has not been a problem until today. If you have any suggestions, I would be happy to try them. I am at the point where I grab onto a straw!