This is by-design and looks like the root cause is the browser.
See these links for reference:
- ASP.NET work item: http://aspnetwebstack.codeplex.com/workitem/188
- Stackoverflow article: Why IEnumerable count is 1 when I upload 0 files? and
- Blog article: http://michaelsync.net/2014/04/29/asp-net-mvc-multiple-files-upload-bug-or-by-design-issue