epub3 format is not supported

[goodlibs-bot]

When downloading books from https://standardebooks.org/opds

Traceback (most recent call last):
  File "site-packages\calibre\gui2\threaded_jobs.py", line 83, in start_work
  File "site-packages\calibre\gui2\ebook_download.py", line 87, in __call__
  File "site-packages\calibre\gui2\ebook_download.py", line 109, in _add
Exception: Not a support e-book format.

It should pick:

1. The default conversion output format
2. The first listed format supported by Calibre.

Alternatively, provide a drop-down menu to select the preferred download format.

The problem here is that the plug-in is just checking the MIME type and not the file extension as well.

Originally posted by @legowerewolf in steinarb#5 (comment)