I found another workaround (sort of). In Preferences > Adding Books > there is a tab for "Adding Actions" and another for "Automatic Adding." Each tab has a checkbox for "Automatically convert added books to output format." If you use only the one in the Automatic Adding tab, then you can add PDFs to the "Ignore books with the following extension" list.
Currently, I have a subfolder (called "Send to Calibre") in my Downloads folder of Win 10. When I get new books, I drop them all into the subfolder and the EPUBs get automagically moved to Calibre and converted to MOBI in one step. PDFs just stay in the subfolder and I add them manually later.
Still having an auto-convert filter would save a step. In case Kovid changes his mind about it... Here's a mockup of what the GUI might look like.
