A ticket from 2011 says this feature was added way back then, but .fs files are not properly highlighted.
Amber Van Heckestaff
Hi Jason, our new source browser shouldn't have issues displaying proper syntax highlighting for F#. You can turn the new browser on via Bitbucket Labs in Settings. I am going to close this issue, but if you see the problem is still occurring please open another ticket so we can work on getting a proper fix.