Fix the issue of returning validation error instead of json.SyntaxError on a required upload file field inside an HTML form

Fix case order hence the fallthrough works like expected.

binding/form_mapping.go

@@ -7,6 +7,7 @@ package binding
 import (
 	"errors"
 	"fmt"
+	"mime/multipart"
 	"reflect"
 	"strconv"
 	"strings"
@@ -157,6 +158,11 @@ func setByForm(value reflect.Value, field reflect.StructField, form map[string][
 			return false, fmt.Errorf("%q is not valid value for %s", vs, value.Type().String())
 		}
 		return true, setArray(vs, value, field)
+	case reflect.Struct:
+		if _, ok := value.Interface().(multipart.FileHeader); ok && value.IsZero() {
+			return false, nil
+		}
+		fallthrough
 	default:
 		var val string
 		if !ok {