Fix: inserting jpg image that are bigger than 90k

Details: when the image is too big, it is saved in a temporary files as-is (meaning raw jpeg data), but the "suffix" is set to "png", and then when loading from the temporary file we force-load as "png", the patch let Qt auto detect the image type

Backport of 0f7fe6dd74c6c78b4e53e9b76c5a96155aeb9241 by myself, reviewd by Boudewijn.