Mapping target for file fields changed to $field_name:uri

The mapping target for file fields changed from "$field_name" to "$field_name:uri". As a consequence, existing Feeds importers that mapped to the file fields' URI will need to be remapped or else the importer will report that the target is "Missing".

A database update function was added to update the importers automatically. Be aware though that if you have put your Feeds importers in code (for example via Features), they will be reported as overridden. In this case re-export your Feeds importer to code or recreate the Feature the importer is in.