Non-image files in the source folder causes failure

[per1234]

It fails if there are any non-images in the source folder. For now I have just noted the limitation in the documentation.

• Try to detect if the file is a valid images after loading
  • gimp-image-is-valid
  • gimp-file-load - maybe use both
    • calling this with an invalid file causes the error so I can't use it
• Filter by extension - this seems to be the only option but I don't like it because it will be lots of code and people may be able to add support for other file types that wouldn't be on the list.