Extension Manager for Mac probabely have a nasty bug.
When I've uploaded my extension (Script optimizer/obfuscator) to Exchange, QA
team said "it is not compatible with Mac".
Screenshot with error was a little strange (something like "can't create
folder", but filename was 100% compatible with Mac!).
Now in next version of my plugin I've found the reason and solution.
Your .mxi file must have _exactly_ the same name as .mxp file!
I don't really know: there is probabely a bug in EM for Mac or EM for Win. Or
maybe I am wrong?