FUZ-19A - Improve testcase handling in Bugzilla Template

[gitstart]

This PR was created by GitStart to address the requirements from this ticket: FUZ-19.

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What is in the PR?

This PR improves how we handle testcase attachments in Bugzilla bug reports. Here's what changed:

1. Made attachment filenames easier to enter:
   • Can now use templates with file extensions from crash testcases
   • Added support for using filename in bug descriptions with {{testcase_attachment}}
2. Added automatic mime-type detection:
   • Used 'mime' npm package to detect correct mime-types from file extensions
   • Falls back to:
     • application/octet-stream (for binary files)
     • text/plain (for text files)

CLIENT DEMO VIDEO: https://www.loom.com/share/af46fd8c8bc140509f71ff5f76ca7c52?sid=22c7c360-64bb-4f5e-81db-13c29da909f7

[community-tc-integration]

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[gitstart-mozilla]

This is a continuation of MozillaSecurity#24