Here's the method that works for me:
- use search with (visibility=invisible) for the invisible folder named 'private' which is located in your Home folder...
[Home]/private/
- open the folder 'private'
- drag the subfolder 'tmp' to your Dock (so you don't have to go thru the above procedure each time)
- temp files are usually found somewhere in the tmp folder, in a subfolder named 'TemporaryItems'
- this can be buried in an invisible subfolder, the name of which may change
I usually copy the file somewhere and then rename the copy with .mov extension -- rather than move or alter the original temp file itself.