You may have found a hole, here.
First, AssetAdmin has never supported replacement of files, so this is an entirely new feature that the DOM is adding to AssetAdmin. The standard AssetAdmin has a custom uploader that, like DOM, sniffs out image types on upload and saves them accordingly.
When replacing a file using the FileIFrameField in the popup, we're using a completely different uploader that needs to be fed the file class ahead of time, e.g.:
$has_one('MyFile' => 'Image');
In this case, the field has all the information it needs to feed the controller the necessary metadata about the type of file being uploaded. What you're saying is that the uploader now needs to be smart enough to ignore the classname that is casted and "upgrade" it to an image when necessary. The DOM bulk uploader, being custom code, is smart enough to do that, but with the replace field, we're dealing with a black box, unfortunately.
Did you in fact get it working? If so show me your changes so I can ratify and add to the module.