Explorer 11 won't play HTML5

[wallypg03]

I'm using UMS to make videos of educational events available to staff who couldn't be at the actual event. Both the video files and UMS are on an old Windows 7 PC I brought in. There are all kinds of IT constraints including Explorer 11 being the only browser on the PCs, and I've had to resort to webm files (created through a VLC conversion from MP4) to make things work with UMS 4.0. With more recent versions of UMS, Flow Player attempts to play HTML5 first which unfortunately fails: 'unsupported video' or variations there of depending on the PC being used. If flash is then selected, no problem playing the webm files. FireFox works without a problem but unfortunately is not, and won't be installed on the work PCs. As far as I can figure out it's to do with the way Explorer 11 handles mime types but I'm not sure how to get UMS to resolve this? By the way, same problem with MP4 files which have been created through FCP. Any advice or direction would be much appreciated!