A few that I've guessed at so far--
- No khakis / chinos, no way. I've seen (a lot) more black or grey jeans in the office. Don't really understand why but that one seems pretty universal.
- Friday is just another day. People don't seem to dress much more casually on Fri than they do Mon - Thurs.
This is for a job in the city and, unfortunately, the answer isn't 'just wear a tie' since tielessness is an important signifier in this line of work.