This functionality is a startup extension to the PDF-Insert/Append and PDF-Attachments tools. It uses a type of Shell extension called Drop handler. This extension is invoked when Shell objects are drag dropped onto a file type registered as drop handler. In our case, the handler is registered to the PDF file type, and the supported Shell objects are the files supported by the respective tools. PDFs, but also .bmp, .png, jpeg, .gif and .tiff images files, when invoking the PDF-Insert/Append tool, and any file type, when invoking the PDF-Attachments tool.
The idea: Insert, append or attach supported document(s) onto a PDF file, by drag dropping it onto that PDF file.
To invoke the Insert/Append tool user just need to drag drop one, or more, supported file type onto a PDF file and the PDF-Insert/Append tool GUI will show up, listing the dropped files as source of pages to insert/append, and the PDF drop target file as target of the insert/append operation.
To invoke the PDF-Attachments tool, the CTRL key must be down when files dropped.
With the ALT key down the tool will run silently, without showing the tool GUI, and using the default tool settings (this functionality is disabled in Trial versions).