fix: csp regressions (#38047)

fix #37257 , all details are in the comments

modules/markup/external/external.go

@@ -48,6 +48,11 @@ func RegisterRenderers() {
 		},
 	})
 
+	markup.RegisterRenderer(&frontendRenderer{
+		name:     "asciicast",
+		patterns: []string{"*.cast"},
+	})
+
 	for _, renderer := range setting.ExternalMarkupRenderers {
 		markup.RegisterRenderer(&Renderer{renderer})
 	}