Before I create a bug report, can someone confirm they also have this problem?
In the Files & Images section, if I switch to Unused files and click on the magnifying glass icon to show the asset (URL http://mysite/admin/assets/EditForm/field/AssetList/item/3/show), I get a popup window with the following message:
"I can't handle sub-URLs of a Form object."
When I close the popup the entire "Unused Files" table disappears!