Beginning with Xojo 2019 R 2.1 am also seeing this, earlier versions open the file as expected.
So: use Xojo 2019 R 1.1 or earlier or use this workaroaund:
[code]Dim app As FolderItem = SpecialFolder.Applications.child(“TextEdit.app”)
Dim document As FolderItem = SpecialFolder.Desktop.child(“APrincessofMars.txt”)
dim sh as new shell
dim command as String = "open -a " + app.ShellPath + " " + document.ShellPath
sh.Execute(command)[/code]