I've searched everywhere and I seem to be the only human on the planet wanting to do this. I've tried all the usual things, making filetypes.Fs.conf, filetypes.fs, editing the configs, what have you. It always thinks .fs is Forth and I cannot make it stop, even deleting the Forth files in Geany or removing the Forth section in the syntax config won't stop it. In addition I can't get F# to appear in the languages list. If it helps, I'm using Windows 7.
F# is not currently supported by Geany. The FAQ points to this page, which is the page about extending Geany, for adding support for other languages.