Hi,
What is your browser ? and do your server use some HtmlWriter component ? or WebControl Adapters ?
I checked the code, and found this:
<button>Upload multiple files</button>
It should not be there.
the uploader should render button like this:
<input type="submit" name="ctl00$ContentPlaceHolder1$Uploader1$_Insert" value="Upload File" id="ctl00_ContentPlaceHolder1_Uploader1__Insert" />
Regards,
Terry