Fixed extracting a zip file issue when uploading files

[janierdavila]

If you select to unzip your file upload today it won't be unzipped.

The bug was in the check for zip extension. It was checking for "zip" instead of ".zip". Path.GetExtension includes the "."

See "http://msdn.microsoft.com/en-us/library/system.io.path.getextension.aspx"