ac3 file extension and passthrough xbmc11

I have an file.ac3 file in 4.0 format (two front and two rear channels). Playing this file I can see the 'Dolby Digital' icon is reported in playing screen so its content is recognized. However, it seems to be mixed down to 2 channels instead using passthrough.

The passthrough itself is working fine for dts files (and they are reported as DTS icon in playing screen).
I still have <dvdplayerignoredtsinwav> in advancedsettings.xml (don't know if this is still needed, it comes from xbmc 10).

I noticed that changing the file extension from file.ac3 to file.dts makes it is not mixed down and it plays using passthrough.
So it looks like the passthrough is selected basing on file extension not by its content (well, at least not only).

Question is (if) how to assign ac3 file extension to be sent as passthrough and not mixed down?

There are two options in Settings > System > Audio for "DTS capable receiver" and "Dolby Digital capable receiver". Are they both checked? Do movies with AC3 audio play via passthrough?

Yes, both options are checked.
The movies with ac3 play fine.

I checked another file with content 5.1 instead 4.0 and there is no difference, means as long as the file extension is .ac3 is is mixed down. Same files renamed to .dts play fine.

I forgot that there is a playercorefactory.xml, where I got:
<rule name="dts/wav" filetypes="(dts|wav)" player="DVDPlayer" />

that obviously needs to be changed to:
<rule name="ac3/dts/wav" filetypes="(ac3|dts|wav)" player="DVDPlayer" />