gistfile1.cs

// Allows us to define our own Dropdown options from code.(UIHint)
        [UIHint("BackgroundPlacementMultiple"), Display(Name = "Background Position", Order = 20, Description = "The background placement for the backgrou image of this block")]
        public virtual string BackgroundPlacement { get; set; }

public class BackgroundSelectionFactory : ISelectionFactory
    {
        public IEnumerable<ISelectItem> GetSelections(ExtendedMetadata metadata)
        {
            var backgroundOptions = new List<SelectItem>();
            backgroundOptions.Add(new SelectItem() { Text = "Top Left", Value = "tl" });
            backgroundOptions.Add(new SelectItem() { Text = "Top Center", Value = "tc" });
            backgroundOptions.Add(new SelectItem() { Text = "Top Right", Value = "tr" });
            backgroundOptions.Add(new SelectItem() { Text = "Center Left", Value = "cl" });
            backgroundOptions.Add(new SelectItem() { Text = "Center Center", Value = "cc" });
            backgroundOptions.Add(new SelectItem() { Text = "Center Right", Value = "cr" });
            backgroundOptions.Add(new SelectItem() { Text = "Bottom Left", Value = "bl" });
            backgroundOptions.Add(new SelectItem() { Text = "Bottom Center", Value = "bc" });
            backgroundOptions.Add(new SelectItem() { Text = "Bottom Right", Value = "br" });

            return backgroundOptions;
        }
    }

    [EditorDescriptorRegistration(TargetType = typeof(string), UIHint = "BackgroundPlacementMultiple")]
    public class BackgroundMultipleEditorDescriptor : EditorDescriptor
    {
        public override void ModifyMetadata(ExtendedMetadata metadata, IEnumerable<Attribute> attributes)
        {
            SelectionFactoryType = typeof(BackgroundSelectionFactory);

            ClientEditingClass = "epi.cms.contentediting.editors.SelectionEditor";

            base.ModifyMetadata(metadata, attributes);
        }
    }