Maybe I'm just not understanding how to use the Read method for the File object, but I see in docs that io.Reader is within the multipart.File interface, but I don't understand how to access it. Any guidance would be greatly appreciated.
That means that the multipart.File interface includes the io.Reader interface, so any object that is a valid multipart.File is also a valid io.Reader. Therefore, you can call the Read method (as defined by io.Reader) on an object of type multipart.File.