I don't think we do that on purpose. The file types that can be uploaded are limited however to make sure no harmful files can be uploaded. It's mostly the well known graphic formats, supported by most software.
I am using Campaign Cartographer 3 and I don't think I can upload the FCW files it generates as its output.
I would suggest zipping your symbol sets as I donk't think we want to add many more custom file types at the moment.