There are a number of similar questions in the Forum. Perhaps reviewing those discussions will help. Start here:
then "search" the Forum for other "Unexpected file format" threads.
The "Search" input box is to the far right.... little magnifying glass... straight across from the "Welcome..."
I've already used the search function and found the topic you've mentioned but no solution is showed there...
Darn!! Well you see that it does happen to others... if you reviewed all those posts, that's about the best I can do.
Since there are so many possible causes unique to your machine, your software, and your project, it's going to be pretty hard for anyone here to do an exact long range diagnoses.
"Best Practices" for Flash developement include "backup often". For many of us, this lesson was learned the hard way!