Preserve preferred extensions when rebuilding MIME data

[OskarEichler]

Summary

Keep each selected MIME type’s own preferred extension when building content_type_mime.db, rather than resolving the row’s current extension back through the global filename index.

The filename lookup can select a different MIME type that shares the extension. With current mime-types-data it cross-selects 49 extension rows, produces 11 wrong preferred-extension records, and returns no type at all for .htaccess, causing rake rebuild_db to abort. This also follows the parity intent discussed in #44.

Verification

• Ruby 4.0.6 via rbenv.
• rake rebuild_db completes against mime-types 3.7.0 / mime-types-data 3.2026.0701.
• All 1,073 generated content-type rows use the exact MIME type’s first extension.
• All 2,684 generated extension/content-type rows resolve exactly through MiniMime.
• The 11 previously cross-selected content types now use their own preferred extensions.
• Existing core suite remains 9 runs / 22 assertions / one identical pre-existing optional parity error for reference-only extensions absent from the repository snapshot.
• ruby -c Rakefile, targeted RuboCop, and source diff checks pass.

No generated database update or test change is included.

[SamSaffron]

fix is good but it needs a test ... cleaning up CI now

[SamSaffron]

sorry addressed in my big pr