Fix - single file files are str only, cast it to list to count properly

[kalisp]

Validate Frames didn't count single file properly. Old implementation expected [], string is provided for single frame.

How to test it:

• try to publish single image file
• see error mesage of Validate Frames

[iLLiCiTiT]

I think single file in list of 'files' will crash during integration... As it's expected it must be a sequence but can't find out what thre frame ranges are. So it should not continue in validation or raise exception that can't publish single file...

EDITED: Unrelated comment