It looks like the HTTP headers were incorrectly added to the file data. First, I recommend determining if this happened during your upload, or your download.
The headers in the screenshot you shared don't seem to match the headers in your upload code, so I suspect the issue using occurring during the download. Are you using custom code to download the file, e.g., as opposed to downloading manually via the web site? If you're downloading via code, it may be good to download from the web site to compare.
In either case, we can't offer help with using whatever third party HTTP client you're using in Go, so you may want to refer to its documentation.
When uploading, the file data should be passed in the HTTP request body, so you'll need to make sure that the HTTP request is properly formatted, with the headers properly separated from the body, and not included in the body itself.
When downloading, the file data is returned in the HTTP response body. When saving the file, you should make sure you only save the response body, and not the response headers. Most HTTP clients offer an easy way to do this, but you may need to refer to the client's documentation to be sure.