Movie File Won't Play in Quick Time After Renaming It

Hi All,

Firstly, I am running Mountain Lion 10.8.1 and Quick Time X 10.2 on an iMac. I have the DivX, Perian and Flip4Mac codecs installed (although after installing Mountain Lion there have been compatibility issues with Perian, which a lot users are having based on the forums I have read).

I have downloaded a TV series which is currently playing fine on Quick Time X 10.2, however as soon as I change the file name from it's original download name:

"Law.and.Order.SVU.S13E01.HDTV.XviD-LOL.[VTV].avi"

to the name that I want to change it to:

"S13E01.avi"

I get the error message:

The document "S13E01.avi" could not be opened. The file may be damaged or may not be a movie file that is compatible with QuickTime player.

The file is compatible with QuickTime as it plays before I changed the file name.

Also, half of the files in this season are ".avi" and the other half are ".mp4". If I change the filename of the ".mp4" it still plays in QuickTime as normal.

It is only the ".avi" files that I am having problems with.

Any ideas how to fix this?

Thanks