Remove :attachment_with_filename feature flag [RUN ALL RSPEC] [RUN AS-IF-FOSS]

What does this MR do?

This MR removes the :attachment_with_filename feature flag that was added in !55066 (merged). This feature flag was enabled in production over a week ago, and no negative impact has been observed.

Does this MR meet the acceptance criteria?

Conformity

• 📋 Does this MR need a changelog?
  • I have included a changelog entry.
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• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Related to #323714 (closed)