how can we check filetypes (formats such as jpg) without using file extensions before _-uploading them-_ in asp.net with c#

hi my dear friends:
how can we check filetypes (formats such as jpg) without using file extensions before -uploading them- in asp.net with c# ?
i am using vs 2008 + asp.net + c# + TELERIK Controls (RadUpload)
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imagine that some body change the text file extension to jpg and select it in a upload conrol such as radupload ...
how can we recognize that this file is truely jpg or not?(In Two Ways -> serverside and clientside)
thanks a lot
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