Uploaded files may pose a significant risk if not handled correctly. A remote attacker could send a multipart/form-data POST request with a specially-crafted filename or mime type and execute arbitrary code.
As you may already know, it is possible to make a website vulnerable to XSS if you can upload/include a SWF file into that website. I am going to represent this SWF file that you can use in your PoCs.
This method is based on [1] and [2], and it has been tested in Google Chrome, Mozilla Firefox, IE9/8; there should not be any problem with other browsers either.
Examples:
Browsers other than IE: http://0me.me/demo/xss/xssproject.swf?js=alert(document.domain);