Is OTHER wrong in the post ? POST : TLDR; my boyfriend accidentally (?) found out my family (and I, by extension) was richer than he thought, I made a comment that didn't help. He got mad/ dejected/ gloomy (?) and I got defensive. AITA?

My family is comfortable funding my 4 years of overseas university education. The university I'm applying to requires proof of finances for foreign students, usually a letter from the bank stating that my parent has x amount with the bank and can afford my education. 

I usually keep these documents at home as I'm scared of losing it but I accidentally brought it to school. I took out that file, opened it, realised it was the wrong file, promptly closed it and put it back in my bag. My boyfriend who was beside me noticed the distinct header of the bank's letter head and asked me what that was. I absent mindedly said "ah just some letter from the bank for uni apps". 

He asked to see it, and I realised what he would be seeing, so I hesitated a bit. He picked up on it and jokingly asked if it's a love letter from some other guy (no malicious intent, it's a running joke). I roll my eyes and hand him the folder and tell him to be careful with it, I need it for applications etc. He looked over it, saw the amount listed in the account and commented something like "wow that's alot". Now, I knew my family was comfortable, but I didn't realise we had this much. I was also shocked by that amount when I first saw the letter. 

So in an effort to be comforting/relatable(?) I said "i know right?" like I was also surprised by that amount. He didn't take well to that, and his mood changed noticeably. I asked him if he's okay, and he said I didn't have to rub it in his face. I got a bit embarrased and annoyed, because I didn't mean for anyone to see those documents, and I tried to back track. He didn't try to communicate anything to me but sit there and gloom with a cloud over his head. I got a bit defensive and told him he asked to see the document in the first place, and what was he expecting anyways? We ignored each other for a while then swept everything under the carpet and pretended it didn't happen (benefits of a HS rs lel). But it's still bugging me. 

 Context re bf: his family situation isn't as stable as mine, but for the rs to work out we agreed really early on to go on dates both of us can afford, and I try not to be an obnoxious bitch about where we can/ cannot go. I have no qualms compromising, and I'm happy with things as they are. I avoid bringing up money as I can sense he feels uncomfortable about it 

