Edited Issue: Validation - consider using FileExtensionsAttribute from System.ComponentModel.DataAnnotations [1059]

The said attribute is new in .net 4.5

We should consider obsoleting out own FileExtensions attribute if the new attribute can be made to work with instances of HttpPostedFileBase (it is currently only for strings)

Otherwise we could consider identifying cases of the .net attribute being set on HttpPostedFileBase properties, and somehow notify the developer about using the correct attribute instead.