A validation rule like the following will fail validation even if the mimetype is the correct one:

    'epub_file':
    {
      required: true,
      accept: "application/epub+zip"
    }

The file I used to test is Orwell's "Animal Farm" from https://www.epubbooks.com/book/1074/animal-farm

After selecting the file and validation failed I ran the following:

document.getElementById('input-epub-upload').files[0]['type']

The output was:

"application/epub+zip"