When you double click on an FLA file it opens the last version of Flash that you used instead of the version of Flash that was used to create the FLA. If you have multiple versions of Flash installed, every click to open an FLA file is like rolling the dice. If you lose you get the "Unexpected file format" message, which really means "open me in the right version of Flash please".
If the last version of Flash you had open was MX 2004 and you double click on a version 6 (Flash MX) FLA file it will open, but it can take a looong time and then you still have to close Flash MX 2004 and reopen the FLA in Flash MX so you can edit it without making a mess of the FLA.
Admittedly the amount of time lost isn't huge, but it sure is annoying and life would be so much easier if the FLA was just opened in the right version of Flash. Well, I finally got fed up and created a utility that will do this... the Flash Version Checker and Launcher or FLAver for short.
When you install FLAver it hijacks the FLA format from Flash and associates all FLA files with FLAver.exe so when you double click on an FLA file it is opened by FLAver first. FLAver doesn't actually open the FLA file for editing, it just looks at the version of the FLA file and at the versions of Flash you have installed and chooses the closest version match for the FLA file. If you try to open an FLA file and you don't have a version of Flash that can open those FLA files FLAVer will present you with an error message like the one below, otherwise the FLA just opens in Flash and you can go on about your business.
FLAver was designed to run on all current versions of the Windows desktop operating systems and works with all versions of Flash from 4 right up to Flash CS4 (Flash 10).
Released: December 2, 2008
Download: flaver.1.2.setup.exe (311KB)