@codebrainz its a good question worth asking, since custom filetypes only cost a menu slot (on a menu that isn't used much usually) I would be happy for lots of specific filetypes, so long as they don't clash with one another. At some time in the future the menu may need re-arranging if its gets too big, but thats a future worry.
In this case the filetype may be based on C/C++, but it has extra features and completely different build commands, so it does seem reasonable for it to be provided as a separate language. And it won't clash with "normal" C++, unlike things like Qt files which still use the normal C++ extensions so they are harder detect, whereas Arduino is using different filetypes, so it will be detected automatically.
The problem with the wiki, is that its a wiki, and its not controlled, remember when your (JSON was it?) filetype was overwritten by one that worked worse. Also I'm afraid many users don't notice whats on the wiki.
@SukkoPera since you have a copyright notice in the filetype file you probably need to provide a license otherwise technically nobody can copy your work. Referring to the GPL v2 as used by the rest of Geany would be best.