mimetype: Make the parsing more resilient
The implementation chosen (try..except) is less clear than checking for the pattern. But that error being not frequent., 2 times so far, I prefer avoidinging the string comparison.
Shout out if you disagree ;)
Related https://sentry.softwareheritage.org/share/issue/518b76d7d7d244f8b88d1757599035b2/
Test Plan
tox
Migrated from D2766 (view on Phabricator)