Fix wrong mimetype for docx files

[gamerlv]

Description

This PR seeks to resolve #1753, allowing files created by the word2007 writer to be detected as word files and not application/octet-stream. I ran into this issue while working on file upload using Laravel's build-in file validation.

Based on this magic file, I've deduced how the word file mimetype is detected.

The file needs to be a zip, inside the first two filenames need to match a regex, then the 3rd and 4th filenames are inspected to see what path they start with. When either matches word/ the file is determined to be a word 2007 file.
Before this process would find docProps/, which does pass the regex, but not match an office mimetype, causing the file to get the type MSOOXML application/octet-stream.

I've added a passing test to ensure this does not pop up again.
I am unsure what documentation needs to be updated for a bugfix.

Fixes #1753

Checklist:

• I have run composer run-script check --timeout=0 and no errors were reported
• The new code is covered by unit tests (check build/coverage for coverage report)
• I have updated the documentation to describe the changes