Which Tiled version? I just tried with the latest development snapshot and couldn’t reproduce the issue. Some notes:
This would not do anything because it would try to open the current file in Tiled, and Tiled just detects that it’s already open and the “current file”.
For me, this started TextEdit, but then TextEdit complained that it didn’t have permission to open the file. No idea why, because the
open -t %mapfile could open the map fine in a text editor.
This command won’t be visible unless you right-click the entry and choose “Execute in Terminal” (or you open the Debug Console window, which can show the command output in the development snapshot version).