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/form.go

@@ -56,5 +56,10 @@ func (formMultipartBinding) Bind(req *http.Request, obj interface{}) error {
 	if err := mapForm(obj, req.MultipartForm.Value); err != nil {
 		return err
 	}
+
+	if err := mapFiles(obj, req); err != nil {
+		return err
+	}
+
 	return validate(obj)
 }