The Edge Proxy does not allow users to embed a valid existing attachment into an HTML Article Body.
Steps to Reproduce
1) Create an edge configuration on kb_knowledge Attachment.
2) Go to kb_knowledge list view and select any record with a filled "Article body" section.
3) Within "Article body" click the "Insert/edit image" mountain icon.
4) Change Type to Attachment.
5) For the image click the paper clip icon and select any valid jpeg or png file.
Observe the error message thrown:
"Error MessageAttempt to save unencrypted data in encrypted attachment for table kb_knowledge"
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The workaround consists in setting Type to "Image Library" instead of "Attachment" in step 4, that allows to embed the image in the HTML body content.
Related Problem: PRB1340117