refactor(IMimeTypeDetector): streamline + minor tidying

[joshtrichards]

Summary

• Simplify code by switching from using FILEINFO_MIME + extra string parsing to outright using FILEINFO_MIME_TYPE
• Sprinkle in OOP-mode usage of finfo
  • P.S. I verified performance/memory usage doesn't suffer from either of the above changes
• More consistent checks
• More consistent variable naming

Unrelated (in terms of this PR itself since it's true even before these changes, but something that came up during testing): PHP v8.4.1-v8.4.10 (current) appear to have dramatically slower finfo performance for some reason (like -2x+).

TODO

• n/a

Checklist

• Code is properly formatted
• Sign-off message is added to all commits
• 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)

[come-nc]

Was the finfo slowness thing reported upstream to PHP?