This is what conkeror outputs when I try to open files (such as .pdf) with an external program (such as
xpdf):

download state change: http://[somefile]: -1, 5, [someid]
download state change: http://[somefile]: 5, 0, [someid]
deleting download: [someid]
download state change: http://[somefile]: 0, 1, [someid]
Error: encountered unknown download

And then, the external program is never started.