Put Mimeloader insertion and read in the same transaction

[tcitworld]

And modernize code a bit.

Checklist

• Code is properly formatted
• Sign-off message is added to all commits
• [x] Tests (unit, integration, api and/or acceptance) are included
• [ ] Screenshots before/after for front-end changes
• [ ] Documentation (manuals or wiki) has been updated or is not required
• [ ] Backports requested where applicable (ex: critical bugfixes)