Wireless router blocking attachments and web forms

I hate to seek out tech advice (especially since I was a tech in my previous life).
This one’s new to me. A few months ago, I installed a Netgear wnr834m wireless router (802.11n). I’ve been reasonably impressed with the throughput and it’s backward compatibility with b and g transceivers from various manufacturers. My roommate, however, tells me that he’s unable to send attached files in email or to submit web-based forms unless he disconnects this router and reconnects the g standard router. What’s got my mind boggled, is that he’s connecting to either router with a CAT5 cable, so the wireless transmission standard should be irrelevant.
I’ve looked through the router configs and nothing is jumping out at me. From my desktop (using an n adapter) and my laptop (which is g standard) I have no problems whatever.
Has anyone got an idea what could cause this?