.m files associated with the C language

[admorgan]

I am sure I am missing the documentation, but looking at the documentation for gcc and clang the .m file is associated with Objective-C (which identify also agrees with), but I can't find a reference that it is used by C.

This is causing me issues when used with a project that contains MatLab files.

[asottile]

hmmm yeah that might be a mistake -- it's been around for quite some time from #12

I can't find a reference for .m being C so I'd be ok with removing that from that extension

cc @chriskuehl in case you have context (though unlikely because it was years ago at this point)

[chriskuehl]

Yeah, I think that is probably a mistake.