multipart/form-data upload receives bad request

I'm trying to upload a file to Canvas using the API. I'm on the second step where you construct the multipart form and POST it but I'm getting a 400 Bad Request error every time with nothing else to ...

uploading a file to Canvas through API

I'm trying to perform a post to POST /api/v1/users/:user_id/files to upload a profile picture for each student. I get an exception in the first step as outlined here: Uploading Files - Canvas L...