Hi, Alex,

You can disassemble the EPiServer dlls in dotPeek and take a peek into the source for MediaData:

[AvailableContentTypes(Availability = Availability.None)]
  [AdministrationSettings(GroupName = "mediatypes", Order = 30, PropertyDefinitionFields = (PropertyDefinitionFields) 65471, Visible = true)]
  public class MediaData : ContentBase, IContentMedia, IContent, IContentData, IBinaryStorable, IRoutable, ILocale
  {
    private CultureInfo _language = CultureInfo.InvariantCulture;
    public const string ChangedByID = "changedByID";
    private string _routeSegment;
    private bool _isModified;
    private Blob _blob;
    private Blob _thumbnail;
    private string _mimeType;

    [UIHint("previewabletext")]
    public string RouteSegment
    {
      get
      {
        return this._routeSegment;
      }
      set
      {
        this.ThrowIfReadOnly();
        this._isModified = true;
        this._routeSegment = value;
      }
    }

    protected override bool IsModified
    {
      get
      {
        if (!base.IsModified)
          return this._isModified;
        else
          return true;
      }
    }

    [Editable(false)]
    public virtual Blob Thumbnail
    {
      get
      {
        return this._thumbnail;
      }
      set
      {
        this.ThrowIfReadOnly();
        this._isModified = true;
        this._thumbnail = value;
      }
    }

    public virtual Blob BinaryData
    {
      get
      {
        return this._blob;
      }
      set
      {
        this.ThrowIfReadOnly();
        this._isModified = true;
        this._mimeType = (string) null;
        this._blob = value;
      }
    }

    public virtual Uri BinaryDataContainer
    {
      get
      {
        return Blob.GetContainerIdentifier(this.ContentGuid);
      }
    }

    public virtual string MimeType
    {
      get
      {
        if (this._mimeType == null && this.BinaryData != null && this.BinaryData.ID.LocalPath != null)
          this._mimeType = MimeMapping.GetMimeMapping(this.BinaryData.ID.LocalPath);
        return this._mimeType;
      }
    }

    public CultureInfo Language
    {
      get
      {
        return this._language;
      }
      set
      {
        this.ThrowIfReadOnly();
        this._isModified = true;
        this._language = value;
      }
    }

    protected override void ResetModified()
    {
      base.ResetModified();
      this._isModified = false;
    }
  }

Then, you can take a look at, for example, ImageData or VideoData.

However, I don't see why you can't extend MediaData? What do you need to do?

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