Validate file extension with magic number using C# and VB.Net in ASP.Net

Hi Sumeet,

For verifying the file extension with magical numbers you need to do workaround.

I have created a sample which full fill your requirement you need to modify the according to your need.

HTML

<div>
    <table border="0" cellpadding="0" cellspacing="0">
        <tr>
            <td>
                <asp:FileUpload ID="fileUpload" runat="server" />
            </td>
        </tr>
        <tr>
            <td>
                <asp:Button ID="Button1" Text="Submit" OnClick="Save" runat="server" />
            </td>
        </tr>
        <tr>
            <td>
                <asp:Label ID="lblMessage" runat="server" />
            </td>
        </tr>
    </table>
</div>

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;
    }
}

VB.Net

Protected Sub Save(sender As Object, e As EventArgs)
    Dim filePath As String = fileUpload.PostedFile.FileName
    Dim filename As String = Path.GetFileName(filePath)
    Dim ext As String = Path.GetExtension(filename)
    Dim contenttype As String = [String].Empty
    Dim checkStream As Stream = fileUpload.PostedFile.InputStream
    Dim chkBinary As New BinaryReader(checkStream)
    Dim chkbytes As [Byte]() = chkBinary.ReadBytes(&H10)

    Dim data_as_hex As String = BitConverter.ToString(chkbytes)
    Dim magicCheck As String = data_as_hex.Substring(0, 11)

    'Set the contenttype based on File Extension
    Select Case magicCheck
        Case "FF-D8-FF-E1"
            contenttype = "image/jpg"
            Exit Select
        Case "FF-D8-FF-E0"
            contenttype = "image/jpeg"
            Exit Select
        Case "25-50-44-46"
            contenttype = "text/pdf"
            Exit Select
    End Select
    If contenttype <> [String].Empty Then
        Dim bytes As [Byte]() = chkBinary.ReadBytes(DirectCast(checkStream.Length, Long))
        lblMessage.Text = Convert.ToString("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
    End If
End Sub

ScreenShot