I searched and didn't find anything about this. I'm not sure if it's a bug or not, it depends on whether there's a reason why mIRC should reject an ignored filetype on a passive send it's making, which doesn't happen on a non-passive dcc-send. It would be nice if mIRC would differentiate between a 'normal' dcc send and the pseudo-send reply coming back from a passive recipient.
When you have dccignore set to only accept a few filetypes, and *.dat is not on the accepted list, you expect to not be able to receive .dat files, but normal dcc-send allows you to send them, but:
/dcc passive on
* Passive DCC is on
/write test.dat test
/dcc send receiver test.dat
DCC Send from receiver rejected (test.dat, file type ignored)
Fails even though the receiver is accepting filetype *.dat