Since the person that maintains this driver (Kolenka) is no longer active and I dont have this device, it is a little hard to debug. Looking at the code, as best as I can tell, the calibre driver creates the mimetype in the books table in books.db. It is set to whatever mimetype is guessed from the file extension for the file on the device.
See
https://github.com/kovidgoyal/calibr...driver.py#L478
What is present in the mime_type column in the books table for books uplaoded by calibre?