How to restrict files with extensions like .exe, .zip etc

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ASP.NET

If i upload a file test.exe, then my application restricts it and give an alert message.
But, if i rename the file as test.exe.txt the application uploads it in database successfully.

How to restrict the upload for test.exe.txt or test.zip.txt?

Please suggest.

Thanks,
Sneha.

What I have tried:

I have tried restricting the contenttype of a file but it is not working as required.

for ex:

If (fuAttachment.PostedFile.ContentType <> "text/HTML" And fuAttachment.PostedFile.ContentType <> "text/plain" And fuAttachment.PostedFile.ContentType <> "Application/msword" And fuAttachment.PostedFile.ContentType <> "image/jpeg" And fuAttachment.PostedFile.ContentType <> "image/GIF" And fuAttachment.PostedFile.ContentType <> "application/pdf" And fuAttachment.PostedFile.ContentType <> "Application/x-msexcel" And fuAttachment.PostedFile.ContentType <> "image/bmp" And fuAttachment.PostedFile.ContentType <> "application/vnd.ms-outlook" And fuAttachment.PostedFile.ContentType <> "image/tiff" And fuAttachment.PostedFile.ContentType <> "video/mp4" And fuAttachment.PostedFile.ContentType <> "application/rar" And fuAttachment.PostedFile.ContentType <> "video/mpeg") Then
                    lblMsg.Visible = True
                    lblMsg.Text = "Upload JPG / BMP / JPEG / TIF / PDF / Doc / Excel / Zip format / MP4 / MPEG / MPG/ RAR / TXT / HTML / MSG files only"
                    Return False
                    Exit Try