ASP.NET generally has a 4MB upload limit by default, so I guess this is the issue.
Try changing the parameter below in your web.config file in the web services folder of TFS - this one should give you a limit of 10MB for uploads.
<httpRuntime maxRequestLength="10240">
Here is link with information on how to change attachment limit http://msdn2.microsoft.com/en-us/library/ms400780.aspx.
Thanks,
Mohammad