I don't think this has been confirmed. And if it was the FBI, it makes no sense why they would place a Bankers Box of framed magazine covers next to them for the photograph, unless that was just an innocent bystander.
My impression from these photos was that the documents came out of that box next to the displayed documents and that trump had stored these documents in boxes with other random items like framed time magazine covers. Didn’t one of the affidavits they released say the docs were randomly mixed in with other papers?
I was thinking that was trump’s super weak effort to hide the items, as if the fbi would open the box, see an old time magazine and think “oh, this is a box of random memorabilia, I’ll just close it back up and move to another box…” or, it might also give trump the super weak excuse of “oh whoops those super secret documents must have just gotten mixed up with my magazines and other junk, sorry guys!”
Instead trump is posting about how his office isn’t usually that messy and that the fbi took the papers and made a mess.