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_mapping.go

@@ -7,6 +7,7 @@ package binding
 import (
 	"errors"
 	"fmt"
+	"net/http"
 	"reflect"
 	"strconv"
 	"strings"
@@ -15,6 +16,34 @@ import (
 	"github.com/gin-gonic/gin/internal/json"
 )
 
+func mapFiles(ptr interface{}, req *http.Request) error {
+	typ := reflect.TypeOf(ptr).Elem()
+	val := reflect.ValueOf(ptr).Elem()
+	for i := 0; i < typ.NumField(); i++ {
+		typeField := typ.Field(i)
+		structField := val.Field(i)
+
+		t := fmt.Sprintf("%s", typeField.Type)
+		if string(t) != "*multipart.FileHeader" {
+			continue
+		}
+
+		inputFieldName := typeField.Tag.Get("form")
+		if inputFieldName == "" {
+			inputFieldName = typeField.Name
+		}
+
+		_, fileHeader, err := req.FormFile(inputFieldName)
+		if err != nil {
+			return err
+		}
+
+		structField.Set(reflect.ValueOf(fileHeader))
+
+	}
+	return nil
+}
+
 var errUnknownType = errors.New("Unknown type")
 
 func mapUri(ptr interface{}, m map[string][]string) error {