C# Mime Types class

you can make use of MediaTypeNames class exists in System.Net.Mime namesapce.

Below is .net class can help you , you dont have to create it by youself.

namespace System.Net.Mime
{
    // Summary:
    //     Specifies the media type information for an e-mail message attachment.
    public static class MediaTypeNames
    {

        // Summary:
        //     Specifies the kind of application data in an e-mail message attachment.
        public static class Application
        {
            // Summary:
            //     Specifies that the System.Net.Mime.MediaTypeNames.Application data is not
            //     interpreted.
            public const string Octet = "application/octet-stream";
            //
            // Summary:
            //     Specifies that the System.Net.Mime.MediaTypeNames.Application data is in
            //     Portable Document Format (PDF).
            public const string Pdf = "application/pdf";
            //
            // Summary:
            //     Specifies that the System.Net.Mime.MediaTypeNames.Application data is in
            //     Rich Text Format (RTF).
            public const string Rtf = "application/rtf";
            //
            // Summary:
            //     Specifies that the System.Net.Mime.MediaTypeNames.Application data is a SOAP
            //     document.
            public const string Soap = "application/soap+xml";
            //
            // Summary:
            //     Specifies that the System.Net.Mime.MediaTypeNames.Application data is compressed.
            public const string Zip = "application/zip";
        }

        // Summary:
        //     Specifies the type of image data in an e-mail message attachment.
        public static class Image
        {
            // Summary:
            //     Specifies that the System.Net.Mime.MediaTypeNames.Image data is in Graphics
            //     Interchange Format (GIF).
            public const string Gif = "image/gif";
            //
            // Summary:
            //     Specifies that the System.Net.Mime.MediaTypeNames.Image data is in Joint
            //     Photographic Experts Group (JPEG) format.
            public const string Jpeg = "image/jpeg";
            //
            // Summary:
            //     Specifies that the System.Net.Mime.MediaTypeNames.Image data is in Tagged
            //     Image File Format (TIFF).
            public const string Tiff = "image/tiff";
        }

        // Summary:
        //     Specifies the type of text data in an e-mail message attachment.
        public static class Text
        {
            // Summary:
            //     Specifies that the System.Net.Mime.MediaTypeNames.Text data is in HTML format.
            public const string Html = "text/html";
            //
            // Summary:
            //     Specifies that the System.Net.Mime.MediaTypeNames.Text data is in plain text
            //     format.
            public const string Plain = "text/plain";
            //
            // Summary:
            //     Specifies that the System.Net.Mime.MediaTypeNames.Text data is in Rich Text
            //     Format (RTF).
            public const string RichText = "text/richtext";
            //
            // Summary:
            //     Specifies that the System.Net.Mime.MediaTypeNames.Text data is in XML format.
            public const string Xml = "text/xml";
        }
    }
}