Rest multipart cannot take list of files.
You can do either of the below. I would recommend the first solution , second solution is a dirty way of doing and it is not scalable.
Solution 1-
Merge all the files you want to send and send it as a single file.
Solution 2 -
Use different attributes for each file.
Anyone who is running into this same issue please upvote the idea posted by Dorus:
https://community.mendix.com/link/space/integrations/ideas/4133