using System.Reflection;
using System.Runtime.InteropServices;

[System.Runtime.InteropServices.DllImport(@"urlmon.dll", CharSet = CharSet.Auto)]
private extern static System.UInt32 FindMimeFromData(
    System.UInt32 pBC,
    [MarshalAs(UnmanagedType.LPStr)] System.String pwzUrl,
    [MarshalAs(UnmanagedType.LPArray)] byte[] pBuffer,
    System.UInt32 cbSize,
    [MarshalAs(UnmanagedType.LPStr)] System.String pwzMimeProposed,
    System.UInt32 dwMimeFlags,
    out System.UInt32 ppwzMimeOut,
    System.UInt32 dwReserverd
);

public void SomeMethod(string fileName)
{
    string mimeType = getMimeFromFile(fileName);

    if (mimeType == "text/plain")
{
        // do something
}
}

private string getMimeFromFile(string filename)
    {
        if (!File.Exists(filename))
            throw new FileNotFoundException(filename + " not found");

        byte[] buffer = new byte[256];
        using (FileStream fs = new FileStream(filename, FileMode.Open))
        {
            if (fs.Length >= 256)
                fs.Read(buffer, 0, 256);
            else
                fs.Read(buffer, 0, (int)fs.Length);
        }
        try
        {
            System.UInt32 mimetype;
            FindMimeFromData(0, null, buffer, 256, null, 0, out mimetype, 0);
            System.IntPtr mimeTypePtr = new IntPtr(mimetype);
            string mime = Marshal.PtrToStringUni(mimeTypePtr);
            Marshal.FreeCoTaskMem(mimeTypePtr);
            return mime;
        }
        catch (Exception ex)
        {
            return ex.Message;
        }
    }

this.dialog = new SaveFileDialog();

            try
            {
                this.dialog.DefaultExt = ".txt";
                this.dialog.Filter = "Text Files|*.txt|Log Files|*.log|All Files|*.*";
                this.dialog.FilterIndex = 2;
            }
            catch ( Exception ex )
            {
                this.tblError.Text = "Error configuring SaveFileDialog: " + ex.Message;
            }