Add change record for file/image target is missing when upgrading from 7.x-2.0-alpha8

Situation: D7 site using feeds 7.x-2.0-alpha8 with some feeds configured to import files and images and some that don't do that.

Client required the deletion of nodes that were no longer in feeds, so the site was upgraded to latest dev (7.x-2.0-alpha8+33-dev) release of Feeds in order to apply patch - https://drupal.org/node/1470530#comment-8777493
Patch worked.

The bug is that either of those updates messed up the feeds that imported files and images.
The node processor mapping turned from "File" to "Missing" at /admin/structure/feeds/XXXXX/mapping

The fix is to create a new mapping for that field, like "File: URI" and fill it with the same info as before and move it to the "missing" one. Same goes for tamper. Then delete the source with missing target.

The only thing that happened was the log got loads of these warnings:
Warning: Invalid argument supplied for foreach() in file_field_presave() (line 218 of /modules/file/file.field.inc)
(and lines 268 and 294).

The cause (not the actual one, just that a mapping was missing) and fix were really hard to track down and I hope this saves someone some headache.