The first file in the ZIP Container MUST be a file by the ASCII name of ‘mimetype’ which holds the MIME type for the ZIP Container (i.e., “application/epub+zip” as an ASCII string; no padding, white-space or case change). The file MUST be neither compressed nor encrypted and there MUST NOT be an extra field in its ZIP header. If this is done, then the ZIP Container offers
When you do a typical Zip, everything gets compressed. There are tricks... But Why?
Why are you editing outside of Sigil? Is it because of the non-TTF/OTF fonts?
Have you tried 'Tweak EPUB' (A version is embedded in Calibre), It takes care of unpackaging- Allowing your edits (and additiions as long as you take care of the OPF changes)-and repackaging into a valid EPUB
The folk at Atlantis have a stand alone version
http://www.atlantiswordprocessor.com...tweak_epub.htm