update attachment module to accept .webp files

[ixc7]

Summary

Closes #2712

Updates the attachment module to accept .webp file uploads

What are the specific steps to test this change?

1. Run the website and log in as an admin
2. Upload any .webp image
3. Save and add the image to a section of the website

What kind of change does this PR introduce?

• [ ] Bug fix
• [x] New feature
• [ ] Refactor
• [ ] Documentation
• [ ] Build-related changes
• [ ] Other

Make sure the PR fulfills these requirements:

• [x] It includes a) the existing issue ID being resolved, b) a convincing reason for adding this feature, or c) a clear description of the bug it resolves
• [ ] The changelog is updated
• [ ] Related documentation has been updated
• [ ] Related tests have been updated

If adding a new feature without an already open issue, it's best to open a feature request issue first and wait for approval before working on it.

Other information:

Testing an upload locally - Screencast

With animated webp - Screencast

Uploadfs with ImageMagick appears to support .webp out of the box without additional changes - Test cases