Originally Posted by Toxaris
I think that you need to manually rename the file to the correct extension. Perhaps a change request to Calbre to have backups renamed to: <filename>_original.epub?
I thought that telling windows to open file type original_epub with sigil as the default program would suffice.
what exactly is sigil objecting to, as the contents of such a file are fine - as proven by the fact that a rename works. but that requires taking a copy from within calibre library - bcause renaming it within the library will mess up calibre - so it's open path, select backup copy, copy to desktop - rename - open - 5 steps.