Change default size of attachments and repo files (#28100) (#28106)

Backport #28100 by @lng2020

https://github.com/go-gitea/gitea/pull/27946 forgets to change them in
code. Sorry about that.

Co-authored-by: Nanguan Lin <70063547+lng2020@users.noreply.github.com>

modules/setting/repository.go

@@ -184,7 +184,7 @@ var (
 			Enabled:      true,
 			TempPath:     "data/tmp/uploads",
 			AllowedTypes: "",
-			FileMaxSize:  3,
+			FileMaxSize:  50,
 			MaxFiles:     5,
 		},