Revert "Merge pull request #1246 from alphagov/65015464-file-attachments-bug"

This reverts commit 25433530811125502f519a7069afe28e1f70f538, reversing
changes made to 3913372c1aafd35ca760f78c185aad0008264048.

We need to hold off from deploying/merging this until the data migration
in f516af324 has been run on production.