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The plugin is not generating thumbnails (it defines a function UtilityController_Thumbnail_Create() but it is never used). As a result, it creates links to non-existent thumb images; some browsers will supply a broken image icon, but some, notably FF, will just ignore the img tag, which makes it look as if no file has been attached to a post.
I tried to insert the UtilityController_Thumbnail_Create() function into PostController_Upload_Create() but without success.