fix attachment file size limit in server backend (#35519)

fix #35512

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Co-authored-by: wxiaoguang <wxiaoguang@gmail.com>

modules/util/error.go

@@ -16,6 +16,7 @@ var (
 	ErrPermissionDenied = errors.New("permission denied")       // also implies HTTP 403
 	ErrNotExist         = errors.New("resource does not exist") // also implies HTTP 404
 	ErrAlreadyExist     = errors.New("resource already exists") // also implies HTTP 409
+	ErrContentTooLarge  = errors.New("content exceeds limit")   // also implies HTTP 413
 
 	// ErrUnprocessableContent implies HTTP 422, the syntax of the request content is correct,
 	// but the server is unable to process the contained instructions