According to this post at volumio.org the Raspberry Pi cannot play 24 bit files via I2S. Instead 24 bit files are converted to 16 bit.
The original post was 4 months ago so maybe this is no longer the case. Of course the HiFiberry DAC does support 24 bit.
I'd appreciate some clarification if 24 bit files are truly supported.
Confusion about 24 bit support