title_body: The mimetype file has an extra field of length n. The use of the extra field feature of the ZIP format is not permitted for the mimetype file
upvoted_answer: I'm responsible for an attempt to improve this error message, see https://github.com/IDPF/epubcheck/pull/497 for details. Essentially your Zip packer puts some application or operating system specific stuff into the file, which breaks EPUB mimetype magic number support.