Ok, then I'm not sure.
I can only think of one thing (if you're prone to the same kind of dumb mistakes as I am). It looks like your comp is hiding known extensions. Are you trying to add the "Project.shp.xml" file (which is appearing as "Project.shp")? It's a longshot, but thought I'd mention it just in case.
Alternatively, could the file have been corrupted? I hope you find a solution!