Xcode: I lost file type on one of my Objective-C .m files-- no more formatting, breakpoints, etc

Having taken no explicit steps that I'm aware of, Xcode has decided that one of my class implementation (.m) files is of unknown file type. So I've suddenly lost all colors/highlighting/autocomplete and (more problematically) the ability for the debugger to recognize breakpoints in that file only.

Right-click on the file and doing Get Info let me forcibly re-select sourcecode.c开发者_JAVA技巧.objc as type, which temporarily restores the syntax highlighting, but it will eventually spontaneously revert to black on white text only. Breakpoints still won't work. Restarting Xcode doesn't clear this state. I cracked open my project.pbxproj file, and made sure that the attributes on that file are the same as all the other .m files. Restarted Xcode again. No dice.

Anyone seen this before and/or can tell me how to forcibly flush this state out?

Thanks!

What happens if you save it under another name?