Upload most recent file to FTP server using PowerShell ISE or WinSCP

There's WinSCP example for your exact task: Upload the most recent file in PowerShell.

The only modification you need to do is that the script is for SFTP, while you want FTP. Though the change is trivial and pretty obvious:

try
{
    # Load WinSCP .NET assembly
    Add-Type -Path "WinSCPnet.dll"
 
    # Setup session options
    $sessionOptions = New-Object WinSCP.SessionOptions -Property @{
        Protocol = [WinSCP.Protocol]::Ftp
        HostName = "example.com"
        UserName = "user"
        Password = "mypassword"
    }
 
    $session = New-Object WinSCP.Session
 
    try
    {
        # Connect
        $session.Open($sessionOptions)
 
        $localPath = "c:\toupload"
        $remotePath = "/home/user"
 
        # Select the most recent file.
        # The !$_.PsIsContainer test excludes subdirectories.
        # With PowerShell 3.0, you can replace this with
        # Get-ChildItem -File switch 
        $latest =
            Get-ChildItem -Path $localPath |
            Where-Object {!$_.PsIsContainer} |
            Sort-Object LastWriteTime -Descending |
            Select-Object -First 1
 
        # Any file at all?
        if ($latest -eq $Null)
        {
            Write-Host "No file found"
            exit 1
        }
 
        # Upload the selected file
        $session.PutFiles(
            [WinSCP.RemotePath]::EscapeFileMask($latest.FullName),
            [WinSCP.RemotePath]::Combine($remotePath, "*")).Check()
    }
    finally
    {
        # Disconnect, clean up
        $session.Dispose()
    }
 
    exit 0
}
catch
{
    Write-Host "Error: $($_.Exception.Message)"
    exit 1
}

If you find anything confusing about the code, you have to be more specific.

Though easier is to use plain WinSCP script with its -latest switch from a plain Windows batch file (or PowerShell if you prefer).