Hi imms,
Please may you check if the extension .ttf has been excluded by your hosting center:
Server side configurations
(section: "Limiting uploadable files", parameter uploadWhiteList).
Otherwise, you should be able to upload.
Moreover, if you don't need to upload manually through the AC resources file manager, you can declare the font in your css file of your webApp, and it will be automatically uploaded (but renamed with MD5 hash name).
Last solution, you can upload directly to the server if you have access to it, put it in res folder.
Regards
J-Serge