test: add coverage for file upload view

[sarkarshrayan2-max]

Closes #410

Summary

Added unit tests for the staff-only file_upload view.

The tests cover:

• Rendering the upload form for staff users
• Uploading a valid PNG image
• Rejecting unsupported file extensions
• Restricting access for non-staff users

Testing

• python manage.py test tests.massmail.views.test_file_upload --keepdb ✅
• python manage.py test tests/ --noinput ⚠️ The full suite has pre-existing failures/errors that were present before this change.

Pull Request Checklist

• Tests passed.

    python manage.py test tests/ --noinput

• No testing required.
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• The documentation is up-to-date

When applicable

• The issue number is provided in the description or title of the pull request.
• Does this close the issue mentioned?
• Screenshots of the results are provided.
• Additional tests have been written.