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 I am using the C# library DotNetZip (Ionic.Zip and Ionic.Zlib) to generate an ebook from a directory. Directory looks like this:
  
  BookName
  |
  |___content/
  |       images/
  |       css/
  |       (html pages, .ops, .ncx)
  |
  |___META-INF/
  |       container.xml
  |
  |___mimetype
  
  
  The code to generate the archive looks like this:
  
  using (ZipFile zip = new ZipFile(pathTemp + ".epub"))
  {
      zip.RemoveSelectedEntries("*.*");
      zip.AddFile(mimetype, "").CompressionLevel = CompressionLevel.None;
      zip.AddDirectory(pathTemp + "\\content", "content");
      zip.AddDirectory(pathTemp + "\\META-INF", "META-INF");
      zip.Save();
  }
  
  
  When I run it through the EPUB Validator, it throws this error:
  
  
    The mimetype file has an extra field of length 36. The use of the extra field feature of the ZIP format is not permitted for the mimetype file.
  
  
  I'm not compressing the mimetype file, so I don't know what is happening.
  
  UPDATE: This problem has been solved; see the solution here.
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.