don't set mime type to text/plain. This will break serving e.g. raw images,

don't set mime type to text/plain. This will break serving e.g. raw images,
html, etc

According to http://golang.org/pkg/net/http/#DetectContentType
"If the Header does not contain a Content-Type line, Write adds
a Content-Type set to the result of passing the initial 512 bytes
of written data to DetectContentType."