So, I’m not sure about the blank response, as it should provide a response, at least it does with Go and curl requests. I was able to use my own Mattermost instance and curl to upload images successfully using the sample curl commands provided on the API documentation page, and it does upload to the server. The important thing to note, however, is that simply uploading to the server does not post the image, it just puts it into storage on the system. In order to actually post the image, you would need to retrieve the image ID from the system and use a webhook to post a message containing the image as an attachment identified either by the image URL to a static location (such as a raw GitHub URL) or the image ID from uploading it to the server. Does this help you at all?