Custom Plugin Attachment Support Enforced + Added Testing

[caronc]

Description:

Related issue (if applicable): #1110

Upon further investigating, the CustomPlugin always has a utf-8 character string; this is confirmed with the added test cases.

One thing to note that before this PR, the Custom Plugin module did not support Attachments (as it should have been allowed). This has been fixed in this PR.

Checklist

• [x] The code change is tested and works locally.
• [x] There is no commented out code in this PR.
• [x] No lint errors (use flake8)
• [x] 100% test coverage

Testing

n/a - more unit tests added in this PR

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 99.28%. Comparing base (22c979d) to head (242207f). Report is 1 commits behind head on master.

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Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #1115 +/- ## ======================================= Coverage 99.27% 99.28% ======================================= Files 139 139 Lines 18053 18056 +3 Branches 3688 3689 +1 ======================================= + Hits 17923 17926 +3 Misses 121 121 Partials 9 9 ```

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