Image and File Fields edited inline do not exhibit 'expected' behaviour

[Lllama]

When inline editing a model with an ImageField the admin application will 'incorrectly' display error messages (for my version of incorrectly). If the parent model already has associated images and the user attempts to upload a new one then an error is produced on the existing child field, stating that a file is required, even though one is already present.

I assume that this is due to the ImageField being validated using only the POST data, when a check should be performed to see whether an image already exists. Under the old core fields method this allowed an instance to be deleted, but since we now have delete buttons I don't see why the additional check can't be made.

[Brian Rosner]

Duplicate of #5721.