Show a preview of blocked/substituted downloads in Get-WinSCP.ps1

The 1MB size-check catches a bad download but gave no way to see why - on a
locked-down network something is very likely substituting a small HTML page
(proxy/AV block page) for the real zip. Print the first lines of whatever
was actually saved so the content is visible without another round trip.

Get-WinSCP.ps1

@@ -21,8 +21,18 @@ try {
 $fileInfo = Get-Item $zip
 if ($fileInfo.Length -lt 1MB) {
     Write-Host "ERROR: Downloaded file is only $($fileInfo.Length) bytes - too small to be WinSCP." -ForegroundColor Red
-    Write-Host "This usually means antivirus or a firewall blocked/altered the download instead of letting the real file through." -ForegroundColor Yellow
-    Write-Host "Try running setup.bat again. If it keeps happening, download WinSCP-6.3.6-Automation.zip manually from winscp.net and unzip it into:" -ForegroundColor Yellow
+    Write-Host "This usually means antivirus, a firewall, or a web filter intercepted the download and replaced it with a block/warning page instead of letting the real file through." -ForegroundColor Yellow
+
+    $preview = Get-Content -Path $zip -TotalCount 15 -ErrorAction SilentlyContinue
+    if ($preview) {
+        Write-Host ""
+        Write-Host "--- What was actually downloaded (first 15 lines, for troubleshooting) ---" -ForegroundColor Yellow
+        $preview | ForEach-Object { Write-Host $_ }
+        Write-Host "--- end ---" -ForegroundColor Yellow
+        Write-Host ""
+    }
+
+    Write-Host "Try running setup.bat again. If it keeps happening, ask IT whether winscp.net / sourceforge.net downloads are being blocked, or download WinSCP-6.3.6-Automation.zip manually in a browser and unzip it into:" -ForegroundColor Yellow
     Write-Host "  $dest" -ForegroundColor Yellow
     Remove-Item $zip -Force
     exit 1