b-man,
For security reason. Uploader do not provide that property .
You can determine the mimetype from the file extension.
Please check our sample code , Ajax-Uploader\AdvancedCS20-MicrosoftAjax\App_Code\SampleUtill.cs
the static public string GetMimeType(string filename) show you how to get the mimetype of a file.
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Maybe we can collect all the relation and provide that property by filename, but we would not allow the value from client.
Regards,
Terry