Fix missing ATTACHMENT__ALLOWED_FILE_EXTENSIONS in readme

[joelvdavies]

Description

See title, was missing before and noticed while creating table for deployment config.

Testing instructions

Add a set up instructions describing how the reviewer should test the code

• Review code
• Check Actions build
• Review changes to test coverage

Agile board tracking

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 83.51%. Comparing base (e0cb04f) to head (b42dab8).

Additional details and impacted files

@@           Coverage Diff            @@
##           develop     #274   +/-   ##
========================================
  Coverage    83.51%   83.51%           
========================================
  Files           25       25           
  Lines          728      728           
========================================
  Hits           608      608           
  Misses         120      120           

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