postWithMultipartResponse works better for uploading this way, per the Slack docs.
POST the contents of your file to the URL returned in step 1. This can be done by sending the raw bytes, or can be a multipart form ended request. If all went well, Slack will respond with an HTTP 200.
Fixes #1290.
postWithMultipartResponse
works better for uploading this way, per the Slack docs.