Hey everyone, after some research I found a solution to this problem. So to anyone who is wondering, I got my solution from this thread here:
HttpRequest upload file - Help / Forum - UiPath Community Forum
Refer to the invoke code described by Niranjan_Mahtole.
The summary of the solution is:
Use invoke code to call the API and attach the file as a binary array to the body, with body format application/octet-stream.