update for supporting file binding (#1264)

update for supporting multipart form and file binding 

example:
```
type PhoptUploadForm struct {
	imgData    *multipart.FileHeader `form:"img_data" binding:"required"`
	ProjectID   string `form:"project_id" binding:"required"`
	Description string `form:"description binding:"required"`
}
```


ref: https://github.com/gin-gonic/gin/issues/1263

binding/binding.go

@@ -98,7 +98,9 @@ func Default(method, contentType string) Binding {
 		return MsgPack
 	case MIMEYAML:
 		return YAML
-	default: //case MIMEPOSTForm, MIMEMultipartPOSTForm:
+	case MIMEMultipartPOSTForm:
+		return FormMultipart
+	default: // case MIMEPOSTForm:
 		return Form
 	}
 }