Automatically chose `type="file"` when generating `<input/>` elements for `IFormFile` expressions

[dougbu]

@Html.Editor() and the <input/> tag helper choose appropriate type attribute values based in part on the given expression's type. these helpers do not currently handle IFormFile or IEnumerable<IFormFile> properties automatically. fixing this is pretty simple and would improve the end-to-end file upload scenario.