Provide Mime type of a file while downloading.

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C#

ASP.NET

What is the best way to provide mime type while downloading a file? I am using a download page which will write binary content and necessary headers to the response stream.
I have tried the following approaches.

1. Using Registry Key

private string GetMimeType(FileInfo fileInfo)
{
    string mimeType = "application/unknown";
    RegistryKey regKey = Registry.ClassesRoot.OpenSubKey(fileInfo.Extension.ToLower());
    if(regKey != null)
    {
        object contentType = regKey.GetValue("Content Type");
        if(contentType != null)
            mimeType = contentType.ToString();
    }
    return mimeType;
}

2. Using urlmon.dll

[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
    );

These two methods were not useful as its not recognizing a lot of extensions from production server. Do you have any other suggestions/thoughts?