AT&T Leads the Way for Windows Phone 7.5 “Mango” Sep 12, 2011 12:31 pm | PC World by Tony Bradley AT&T has unveiled a lineup of new Windows Phone 7.5 “Mango” smartphones coming soon, and plans to upgrade existing Windows Phone 7 devices to Mango soon. With the additional devices, AT&T will have seven different Windows Phone 7 smartphones to choose from.   AT&T currently offers four different Windows Phone…