62e59e6250
The exists? method in post/file has a different implementation for
PSH sessions than other shells which are testing for the existence
of a path, not the presence of a file.
Fix this by replacing [System.IO.File]::Exists with Test-Path.
Testing:
```
PS C:\Windows\system32> [System.IO.File]::Exists("C:\")
False
PS C:\Windows\system32>test-path C:\
PS C:\Windows\system32> test-path C:\
True
```