Upset because of the medical stuff asked

Time to change and buy a new car. I was delivering the papers they normally ask you for giving me the credit: Last payment of taxes, payslip, personal identification and such.
In my case, I don’t have a normal payslip but a document where appear all the details referred to my retirement. Since I had been retired from my job for medical reasons.

My surprise is that they need to have also the document where the medical resolution appeared and I gave it also to them. It seems it’s not enough, they need to know which desease I was retired for.
I don’t mind if I voluntarily want to offer that information but being forced it’s pretty upsetting.
Indeed, that information doesn’t appear in any of the official papers I received, only in my psychiatrist report, that it was previous to my pension granted.
I understand they have to minimise the risk but they already have enough information, they know I have been only retired for my job and I don’t need another person to cope with my daily activities. What else they want?
I’m not gonna give them more information. It’s not a matter of me wanting to hide anything, it’s a matter of dignity.