That certainly ain't a good thing that iMovie is attempting to opening them up. Dpending upon which version iMovie you have it's import functions are fairly limited!

To make default apps for certain types of files:

(1) I must be just over-tired at the moment but I can't think of the approp method. One technique is to use a file utility (I use mac army knife) to change the file attributes so that the Creator of the file matches that of the app I wish to open it with.

(2) Alternatively, (& probably the best way) is to highlight a sample of that type of file & then CMD + I, then go to the 'Open with' part & choose your App. This should fix the prob. Also it might be wise to rebuild your desktop with whatever utilities you have that can do it. Here's an example: Desktop Rebuilder for OSX Hope this helps! If not, post back. Good luck!