Hi Johan,
ok, I understand what is happening. Actually it's somewhat surprising that FireFox shows it in the media control.
For security reasons files served from the /uploads folder always have their content-Disposition set to "attachment". Meaning that the browser should not display, or run, the file, but should instead treat it as a download.
This is because users can upload stuff to the server, and those files end up in /uploads.
So you should move the file out of uploads into something else (which is why it worked from the Reports folder) once you are sure the file is ok.
Cheers
Bruce