I get the exact same error when using system.importTag, were you able to find a solution?
Try this line in your code:
curl_setopt($ch, CURLOPT_POSTFIELDS, http_build_query( array("uploadFile" =@c:\reports\test.xml ) ) ); // add POST fields
I had alot of issues with this one, my URL now looks like this:
Its the "@" that does the trick.
Hope that helps.