I\'ve been following this tutorial to support file uploading as part of MVC4\'s WebAPI: http://blogs.msdn.com/b/henrikn/archive/2012/03/01/file-upload-and-asp-net-web-api.as
Can you try adding the "name" attribute to your input file?
<input name="file1" id="file1" type="file" multiple="multiple" />