Suppress epub warnings about unknown mimetype

[parulin]

Fix build error introduced by my previous commit (a16b256)

See: #1574 (comment)

[rapenne-s]

Any idea how to test the fail_on_warning with the same building process they use on rtd?

ping @maiska

[parulin]

Reading the logs that's unclear if fail_on_warning really fails or if it follows the build process only to exit with an error?

From the docs about sphinx.fail_on_warning:

Turn warnings into errors (-W and --keep-going options). This means the build fails if there is a warning and exits with exit status 1.

So testing it on production should have no impact on the user's side?