It would be nice to have option "execute custom command" in right-click context menu in downloads (transfers) list. It can be configurable similar to command on file completion, but pool of variables to pass should be larger - add path to met file for incomplete, full target file name, hash of course, last reception time, files status, number of sources etc.
Thakns to above, I could pass data of file that is either complete or not, to my external handler and try to view file with my favourite applications, query my custom database of downloaded files and make decision whether I want to download or to cancel a file, and many other interesting things.