From this answer at
askubuntu.com which refers to some
freedesktop.org spec, I gather what you're looking for is possible by creating a
.desktop file with an
EXEC= entry. I suppose the recognized field codes (e.g.
%u) get replaced with the dropped file's URI/path:
%f A single file name, even if multiple files are selected. The system reading the desktop entry should recognize that the program in question cannot handle multiple file arguments, and it should should probably spawn and execute multiple copies of a program for each selected file if the program is not able to handle additional file arguments. If files are not on the local file system (i.e. are on HTTP or FTP locations), the files will be copied to the local file system and
%f will be expanded to point at the temporary file. Used for programs that do not understand the URL syntax.
%F A list of files. Use for apps that can open several local files at once. Each file is passed as a separate argument to the executable program.
%u A single URL. Local files may either be passed as file: URLs or as file path.
%U A list of URLs. Each URL is passed as a separate argument to the executable program. Local files may either be passed as file: URLs or as file path.
(...but I've neither ever tried this nor do I know if it's supported by KDE, so someone knowledgeable pitching in would be good.)
There might be other ways, via Plasma starter widgets, but I have no idea.