fix(embed): Allow attachment protocols for thumbnails and images

[Jiralite]

Please describe the changes this PR makes and why it should be merged:
Embed thumbnails and images may use the attachment:// protocol. Icon URLs and footer icon URLs are currently fine as-is.

Status and versioning classification:

• Code changes have been tested against the Discord API, or there are no code changes
• I know how to update typings and have done so, or typings don't need updating

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2 Skipped Deployments

Name	Status	Preview	Comments	Updated (UTC)
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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 39.15%. Comparing base (e273afb) to head (b25d01f).
Report is 2 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main   #10795   +/-   ##
=======================================
  Coverage   39.15%   39.15%           
=======================================
  Files         244      244           
  Lines       14804    14804           
  Branches     1413     1413           
=======================================
  Hits         5796     5796           
  Misses       8996     8996           
  Partials       12       12           

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