Read content type from JSON map if it exist

refs SkygearIO/skygear-SDK-JS#164
refs SkygearIO#287

pkg/server/skydb/skyconv/map.go

@@ -122,8 +122,17 @@ func (asset *MapAsset) FromMap(m map[string]interface{}) error {
 	if name == "" {
 		return errors.New("asset's $name should not be empty")
 	}
-
 	asset.Name = name
+
+	contentTypei, ok := m["$content_type"]
+	if ok {
+		contentType, ok := contentTypei.(string)
+		if !ok {
+			return fmt.Errorf("got type($contentType) = %T, want string", contentTypei)
+		}
+		asset.ContentType = contentType
+	}
+
 	return nil
 }