A debt collector sent me a letter with the creditor's name and my related file number visible through the clear plastic window; is this legal?

(This a throwaway reddit account for privacy reasons.)

I had a credit card account that was eventually sent off to a debt collector. Said debt collector sued me, we settled out of court by setting up a payment plan.

Checking the mail today, I had a letter from them. I have not opened the letter yet. I noticed that the name of my original creditor was very clearly visible in the corner of the plastic window. Curious if this was the only information readily available without opening the envelope, I flipped it over and gave the edge a tap on my desk to settle the paper against the top edge. Doing so left the following visible:

RE: Creditor: <Name of creditor>

Our File No.: <My file number with the debt collector>

This strikes me as sensitive information that should be kept private, shouldn't it? And if so, what should I do in this situation?

Edit: Apologies, forgot to include the location. I myself am from the US, specifically the state of New Jersey. The debt collector is from Pennsylvania.