Use DetectContentType to detect the uploaded file content-type, but the result is always application/octet-stream.this is just the default return in this function.
So what's the problem?
The standard library's code is only supposed to detect certain types (like HTML that contains one of a few common tags) according to a certain standardized algorithm--more in https://golang.org/src/net/http/sniff.go. You could use github.com/rakyll/magicmime, which uses libmagic and cgo to guess at more file types.