By comparing with the DAMP source code (DAMP) I noticed that the new MediaType class Umbraco.Core.Models.MediaType is missing the Save() method, so no matter how I tested, it simply would not save the new media type.

Anyway, also by using DAMP's example (good old copy/paste), that uses the old assemblies and shows a message 'OBSOLETE', came up with a new version of the code, which worked as expected.

So, this is possibly something that I'll have to revisit in the near future.

What the new code looks like: using System; using System.Drawing; using System.Drawing.Drawing2D; using System.Drawing.Imaging; using System.IO; using System.Xml; using umbraco.BusinessLogic; using umbraco.cms.businesslogic.datatype; using umbraco.cms.businesslogic.datatype.controls; using umbraco.cms.businesslogic.media; using umbraco.cms.businesslogic.packager.standardPackageActions; using umbraco.DataLayer; using umbraco.interfaces; using umbraco.IO;

namespace SocialMediaChannels
{
    public class AddSocialMediaThemes : IPackageAction
    {
        private readonly static string MediaTypeName = "Social Media Theme";
        private readonly static int LABEL_ID = -92;
        private readonly static int UPLOAD_ID = -90;
        private readonly static int TEXT_ID = -88;
        private readonly static int DATE_ID = -41;
        private readonly static int IMAGE_ID = 1031;
        private readonly static int FOLDER_ID = 1032;

        public IDataType uploadField = new Factory().GetNewObject(new Guid("5032a6e6-69e3-491d-bb28-cd31cd11086c"));


        protected static ISqlHelper SqlHelper
        {
            get
            {
                return umbraco.BusinessLogic.Application.SqlHelper;
            }
        }
        public string Alias()
        {
            return "SocialMediaChannels_AddSocialMediaThemes";
        }

        public bool Execute(string packageName, XmlNode xmlData)
        {
            bool flag;

            try
            {
                #region MediaType
                User adminUser = new User(0);

                MediaType theme = CreateMediaType(adminUser, MediaTypeName);

                MediaType folder = MediaType.GetByAlias("Folder");

                int[] folderStructure = folder.AllowedChildContentTypeIDs;
                int newsize = folderStructure.Length + 1;

                Array.Resize(ref folderStructure, newsize);
                folderStructure[newsize - 1] = theme.Id;

                folder.AllowedChildContentTypeIDs = folderStructure;

                MediaType image = MediaType.GetByAlias(MediaTypeName);


                #endregion

                #region Theme Images
                #endregion

                flag = true;
            }
            catch
            {

                flag = false;
            }
            return flag;

        }

        public bool Undo(string packageName, XmlNode xmlData)
        {
            bool flag;
            try
            {
                //remove themes
                flag = true;
            }
            catch
            {
                flag = false;
            }
            return flag;
        }

        public XmlNode SampleXml()
        {
            string sample = string.Format("<Action runat=\"install\" undo=\"true/false\" alias=\"{0}\" ></Action>", Alias());
            return helper.parseStringToXmlNode(sample);
        }
        /// <summary>
        /// Create a Media Type.
        /// </summary>
        /// <param name="adminUser"></param>
        /// <param name="mediaTypeName"></param>
        /// <returns></returns>
        private MediaType CreateMediaType(User adminUser, string mediaTypeName)
        {
            MediaType mediaType = MediaType.MakeNew(adminUser, mediaTypeName);
            int[] typeIds =  { FOLDER_ID, IMAGE_ID };
            mediaType.AllowAtRoot = true;
            mediaType.Text = MediaTypeName;
            mediaType.Description = "Container for the Social Media Channel Theme Images";
            mediaType.IsContainerContentType = true;
            mediaType.AllowedChildContentTypeIDs = typeIds;

            //Add properties
            mediaType.AddPropertyType(new DataTypeDefinition(UPLOAD_ID), "umbracoFile", "Upload image");
            mediaType.AddPropertyType(new DataTypeDefinition(LABEL_ID), "umbracoWidth", "Width");
            mediaType.AddPropertyType(new DataTypeDefinition(LABEL_ID), "umbracoHeight", "Height");
            mediaType.AddPropertyType(new DataTypeDefinition(LABEL_ID), "umbracoBytes", "Size");
            mediaType.AddPropertyType(new DataTypeDefinition(LABEL_ID), "umbracoExtension", "Type");

            mediaType.AddPropertyType(new DataTypeDefinition(TEXT_ID), "themeName", "Name of the Social Channel Theme");
            mediaType.AddPropertyType(new DataTypeDefinition(TEXT_ID), "themeUrl", "Url for the Theme");
            mediaType.AddPropertyType(new DataTypeDefinition(TEXT_ID), "createdBy", "Author of the Theme");
            mediaType.AddPropertyType(new DataTypeDefinition(DATE_ID), "createdDate", "Date the Theme was created");

            mediaType.Text = mediaTypeName;
            mediaType.IconUrl = "mediaPhoto.gif";
            mediaType.Save();

            return mediaType;
        }
    }
}

Turns out there were a lot of things wrong with my original code. Finally got it working after lots of investigation.

Basically the Adding (which was causing it to fail) became:

    public static void AddMediaType()
    {
        MediaType mediaType = new MediaType(-1);
        mediaType.AllowedAsRoot = true;
        mediaType.Name = NAME;
        mediaType.Description = "Container for the Social Media Channel Theme Images";
        mediaType.IsContainer = true;
        mediaType.Icon = "icon-picture";
        mediaType.Alias = ALIAS;

        //Allowed child nodes
        var children = new List<ContentTypeSort>
            {
                new ContentTypeSort(FOLDER_ID, 0),
                new ContentTypeSort(IMAGE_ID, 1)
            };

        mediaType.AllowedContentTypes = children;
        DataTypeService dataTypeService = (DataTypeService)ApplicationContext.Current.Services.DataTypeService;


        //Add properties
        var name = new PropertyType(dataTypeService.GetDataTypeDefinitionById(TEXT_ID), "themeName");
        name.Name = "Theme Name";
        name.Description = "Name for the theme";
        name.SortOrder = 0;

        var url = new PropertyType(dataTypeService.GetDataTypeDefinitionById(TEXT_ID), "themeUrl");
        url.Name = "Theme Url";
        url.Description = "Url for the original theme";
        url.SortOrder = 1;

        var createdBy = new PropertyType(dataTypeService.GetDataTypeDefinitionById(TEXT_ID), "createdBy");
        createdBy.Name = "Created By";
        createdBy.Description = "Theme Author";
        createdBy.SortOrder = 2;

        var createdDate = new PropertyType(dataTypeService.GetDataTypeDefinitionById(DATE_ID), "createdDate");
        createdDate.Name = "Created Date";
        createdDate.Description = "Date the Theme was created";
        createdDate.SortOrder = 3;

        mediaType.AddPropertyType(name, "Image");
        mediaType.AddPropertyType(url, "Image");
        mediaType.AddPropertyType(createdBy, "Image");
        mediaType.AddPropertyType(createdDate, "Image");

        ContentTypeService contentTypeService = (ContentTypeService)ApplicationContext.Current.Services.ContentTypeService;
        contentTypeService.Save(mediaType);
    }

And now it works!

is working on a reply...