Fix returns invalid Content-Type on serving static file

Kocha made invalid Content-Type response.
This cause is http.DetectContentType() has not good behavior.
ex)
javascript -> text/plain
SVG -> text/xml

http.DetectContentType() checks content body only, but I think path extension is more important.
So, I changed to use mime.TypeByExtension() before http.DetectContentType() on this PR.

thank you!