The media files in the content .opf file should be media-type="application/xhtml+xml". However, sometimes my PDF converter produces ePub files that are of type "text/html" and this generates an error. I wanted to add something to the plugin that I am developing (see
https://www.mobileread.com/forums/sho...d.php?t=264378) so that I can do a search and replace for these errors.