It's notable Discord was developed as an American startup, and it's not clear what Tencent's involvement is. Regardless, for me it's too much access for a chat app to have in exchange for the convenince of sharing a photo from my phone.
Why do I need to give snapchat access to all photos ever just to post from my camera roll?
If you want to try it out install the Wire messenger (if you make the account with a web browser you don't need to provide a phone number), and try to attach a photo but deny Photo library permissions. (Here's the buttons to press: https://imgur.com/a/gc5Iq). Other apps work this way on iOS 11 but this is the one that came to mind.