Cannot delete attached file from invoice

Overview: Cannot delete attached file from invoice. when editing the invoice you are able to use the trash can icon to delete the file. However after save and close invoice the file still appears to be attached.

Steps to Reproduce:

1. Log into bugfest-iris
2. Create invoice and invoice line
3. Attach a file to the invoice
4. Save and close
5. Edit invoice again
6. Remove the attached file
7. Click save and close
8. Expand "Linked documents" accordion

Expected Results: Invoice is saved successfully AND file no longer appears as attached.

Actual Results: Invoice is saved successfully and file STILL appears as attached to invoice.

Additional Information: https://foliotest.testrail.io/index.php?/tests/view/665296
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