fix: restrict mime type list depending on check operator

[R0Wi]

• Resolves: Workflowengine "The given regular expression is invalid" #23666

Summary

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)

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[R0Wi]

@blizzz I rebased this against the current master and adjusted to code to work with c24ead8.

Would be happy to see a review of this since me and other users are stumbling across the described issue regularly 😄

[R0Wi]

@susnux may I kindly ask you for a review, please?