Unfortunately no. The uploader can only filter by file extension. But you can implement your own filter mechanism based on this propery:

http://www.essentialobjects.com/ViewDoc.aspx?t=EO.Web.AJAXPostedFile.ContentType.html

The key is, unlike filtering by file extension which is done on the client side before any file is transferred to the server, filtering based on ContentType (MIME type) is done on the server side after all files have been transferred to the server. The reason is that user can upload multiple files with different MIME types in a single upload. The server side can not accept part of the request and reject some other part of the request. So all files always get transferred to the server side, once they are all there, you can then decide whether you want to abandon some of them based on the ContentType property.

Thanks