This happens with the "Copy Cover" and ¨"Paste Cover" features in the cover box.
For example, I want to copy a 27KB image/jpg 301x300 cover and paste it to another file.
When I paste the cover this copies as a 66KB image/jpeg 301x300.
Main differences: file size (from 27KB to 66KB) and file type(from image/jpg to image/jpeg).
Any help is greatly appreciated.