Hi Quick note: Please do not share API tokens. They should be kept secret at all times. I’ve gone ahead and expired the one you posted so you’ll need to generate a new one.
Not going to lie, attachments are tricky to work with over REST. I put together this project to help show different ways of interacting with the attachments endpoints: Glitch :･ﾟ✧
There is also this question and solution from awhile back that might point you in the right direction: Attaching PDF attachments fail in the API POST call - #2 by bentley