I've just created my first custom field type and uploaded the dll to the server however it is not currently available for selection. I followed the developer docs and a copy of my class can be seen below - did I miss something obvious?

namespace ProlificNotion.Umbraco.Contour.FieldTypes
{
    using System;
    using System.Collections.Generic;
    using System.Globalization;
    using System.Web.UI.WebControls;

    using global::Umbraco.Forms.Core;

    /// <summary>
    /// Defines the CountryList FieldType for Umbraco Contour
    /// </summary>
    public class CountryDropDownList : FieldType
    {
        /// <summary>
        /// The dropdown list control
        /// </summary>
        private readonly DropDownList ddl;

        /// <summary>
        /// Initializes a new instance of the <see cref="CountryDropDownList"/> class.
        /// </summary>
        public CountryDropDownList()
        {
            this.Id = new Guid("35EF2032-6F72-4C00-9515-2D93C27AA574");
            this.Name = "Country List";
            this.Description = "Renders an HTML select box with a list of countries";

            this.Icon = "icon_field.gif";
            this.DataType = FieldDataType.String;

            this.ddl = new DropDownList();
            this.FillWithISOCountries(this.ddl, CultureInfo.InvariantCulture);
        }

        /// <summary>
        /// Gets or sets the editor.
        /// </summary>
        /// <value>
        /// The editor.
        /// </value>
        public override WebControl Editor
        {
            get
            {
                return this.ddl;
            }

            set
            {
                base.Editor = value;
            }
        }

        /// <summary>
        /// Renders the preview.
        /// </summary>
        /// <returns>A string with the markup to render in the preview</returns>
        public override string RenderPreview()
        {
            return "<select><option>United Kingdom</option></select>";
        }

        /// <summary>
        /// Renders the preview with prevalues.
        /// </summary>
        /// <param name="prevalues">The prevalues.</param>
        /// <returns>A string with the markup to render in the preview</returns>
        public override string RenderPreviewWithPrevalues(List<object> prevalues)
        {
            return this.RenderPreview();
        }

        /// <summary>
        /// Populates the list control with countries as given by ISO 4217.
        /// </summary>
        /// <param name="ctrl">The list control to populate.</param>
        /// <param name="selectedCulture">The selected culture.</param>
        private void FillWithISOCountries(ListControl ctrl, CultureInfo selectedCulture)
        {
            foreach (var cultureInfo in CultureInfo.GetCultures(CultureTypes.SpecificCultures))
            {
                var regionInfo = new RegionInfo(cultureInfo.LCID);
                if (ctrl.Items.FindByValue(regionInfo.TwoLetterISORegionName) == null)
                {
                    ctrl.Items.Add(new ListItem(regionInfo.EnglishName, regionInfo.TwoLetterISORegionName));
                }
            }

            var currentRegionInfo = new RegionInfo(selectedCulture.LCID);

            // Default the selection to the current cultures country
            if (ctrl.Items.FindByValue(currentRegionInfo.TwoLetterISORegionName) != null)
            {
                ctrl.Items.FindByValue(currentRegionInfo.TwoLetterISORegionName).Selected = true;
            }
        }
    }
}