Add 'files' as a reserved team (#16823)

Co-authored-by: Mattermod <mattermod@users.noreply.github.com>

model/utils.go

@@ -394,6 +394,7 @@ var reservedName = []string{
 	"channel",
 	"claim",
 	"error",
+	"files",
 	"help",
 	"landing",
 	"login",