FileUpload ContentType trickery

I am developing an app where a user uploads PDF files to the server, via a FileUpload control.
As part of the usual security/reliability checks I wanted to verify that the uploaded document was actually a PDF, not just say, a Text file renamed to have a .PDF extension.
I thought this might work:
If Not FileUploadControl.PostedFile.ContentType.ToLower = "application/pdf" Then ltl_status.Text = "<p>Error - The upload file must be in PDF format.</p>" Exit Sub End IfThe above code however is fooled if I rename a Text file with a PDF extension and upload it. Is there anyway to actually check the real type of the file instead of just presuming the file extension is accurate?
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