Just a thought for those who may want to go the route of unzip - edit - re-zip their epubs.
I'm so old I remember doing this routinely, in the era Before Sigil.
If I recall correctly, the re-zipping has to be done in a particular way such that the "mimetype" file ends up as the first file in the root of the zip/epub directory, and it must be exactly 20 bytes in size, and it must have no compression. The content of the mimetype file is simply "application/epub+zip" (without quotes).
Again, IIRC, I did it by first zipping the mymetype file alone into a fresh zip file, then add the META-INF and OEBPS subdirectories in another step(s). (did that have to be done separately, in that order? I don't recall.)
Someone correct me if I've remembered it wrong. At the time, I just wrote a script file to handle it so I wouldn't have to think about it. Now, I've lost the script file.
Moral of the story: a very big
to the Sigil developers and maintainers.
Albert