Support downloading remote tarball contexts in builder jobs.

Signed-off-by: Moysés Borges <moysesb@gmail.com>

httputils/mimetype.go

@@ -0,0 +1,30 @@
+package httputils
+
+import (
+	"mime"
+	"net/http"
+)
+
+var MimeTypes = struct {
+	TextPlain   string
+	Tar         string
+	OctetStream string
+}{"text/plain", "application/tar", "application/octet-stream"}
+
+// DetectContentType returns a best guess representation of the MIME
+// content type for the bytes at c.  The value detected by
+// http.DetectContentType is guaranteed not be nil, defaulting to
+// application/octet-stream when a better guess cannot be made. The
+// result of this detection is then run through mime.ParseMediaType()
+// which separates it from any parameters.
+// Note that calling this function does not advance the Reader at r
+func DetectContentType(c []byte) (string, map[string]string, error) {
+
+	ct := http.DetectContentType(c)
+	contentType, args, err := mime.ParseMediaType(ct)
+	if err != nil {
+		return "", nil, err
+	}
+
+	return contentType, args, nil
+}