Some attachments in Test steps have "Preview unavailable" after migration

Attachments referenced using a relative path in the Test steps are not processed correctly during the migration, resulting in a "Preview unavailable" warning after migrating to the Cloud.

Steps to reproduce:

1 - In the DC site, create a Test.
2 - Create a Test step and add/paste an attachment.
3 - Switch to the Text edit mode, and edit the markdown so that the attachment has a relative path, e.g.:

!plugins/servlet/raven/attachment/12345/my-image.png|width=300,height=137!

4 - Save the Test step and confirm that the attachment is previewed correctly in the Test step in the DC.
5 - Migrate to Cloud.

Expected result: The attachment is preview correctly in the migrated Test step in the Cloud site.

Actual result: The attachment is displayed with a "Preview unavailable" warning in the migrated Test step in the Cloud site.