Validate file type (extension) using magic number in ASP.Net

I check fileupload magicCheck before uploading.

Scenario 1 :- I change my .exe file to .pdf when I Upload that file I am getting error message that is (Correct)

Scenario 1 :- I upload same file and change the request before uploading from tool (burpSuit) change its mime type .exe file(MZ) to the magic number .pdf file (%PDF-) and forward that request it will Upload how to restrict that.

C#

protected void Save(object sender, EventArgs e)
{
    string filePath = fileUpload.PostedFile.FileName;
    string filename = Path.GetFileName(filePath);
    string ext = Path.GetExtension(filename);
    string contenttype = String.Empty;
    Stream checkStream = fileUpload.PostedFile.InputStream;
    BinaryReader chkBinary = new BinaryReader(checkStream);
    Byte[] chkbytes = chkBinary.ReadBytes(0x10);
    string data_as_hex = BitConverter.ToString(chkbytes);
    string magicCheck = data_as_hex.Substring(0, 11);
    //Set the contenttype based on File Extension 
    switch (magicCheck)
    {
        case "FF-D8-FF-E1": contenttype = "image/jpg";
            break;
        case "FF-D8-FF-E0": contenttype = "image/jpeg";
            break;
        case "25-50-44-46": contenttype = "text/pdf";
            break;
    }
    if (contenttype != String.Empty)
    {
        Byte[] bytes = chkBinary.ReadBytes((Int32)checkStream.Length);
        lblMessage.Text = "Uploaded File is of type " + contenttype;
        lblMessage.ForeColor = System.Drawing.Color.Green;
    }
    else
    {
        lblMessage.Text = "Please Upload Valid File with Original extension";
        lblMessage.ForeColor = System.Drawing.Color.Red;
    }
}