This may come obvious to some, but apparently a lot of people seem to be making this mistake.
To ensure the safety of the file downloading, when you test it with avipreview;
1. copy the file out of MySharefolder(You may also want to pause the download while doing this)
2. copy it to the desktop and rename it to something else. (eg. movie)
3. open avipreview and open the copied file from the desktop and ther you go
If you use this method you will not corrupt the file; if you don't use this method you could corrupt the file, because as your testing the file, you are appending to that file.