fix(binding): binding error while not upload file (#3819) (#3820)

Co-authored-by: zhangmj <zhangmj1@dustess.com>

binding/form_mapping_test.go

@@ -5,6 +5,7 @@
 package binding
 
 import (
+	"mime/multipart"
 	"reflect"
 	"testing"
 	"time"
@@ -43,6 +44,7 @@ func TestMappingBaseTypes(t *testing.T) {
 		{"zero value", struct{ F uint }{}, "", uint(0)},
 		{"zero value", struct{ F bool }{}, "", false},
 		{"zero value", struct{ F float32 }{}, "", float32(0)},
+		{"file value", struct{ F *multipart.FileHeader }{}, "", &multipart.FileHeader{}},
 	} {
 		tp := reflect.TypeOf(tt.value)
 		testName := tt.name + ":" + tp.Field(0).Type.String()