Implementing GetEnumerator() for a collection inherited from List<string>

I am trying to implement FilePathCollection. Its items would be simple file names (without a path - such as "image.jpg"). Once the collection is used via foreach cycle, it should return the full path created by concatenating with baseDirectory. How can I do that?

public class FilePathCollection : List<string>
{
    string baseDirectory;

    public FilePathCollection(string baseDirectory)
    {
        this.baseDirectory = baseDirectory;
    }

    new public System.Collections.IEnumerator GetEnumerator()
    {
        foreach (string value in this._items) //this does not work because _list is private
            yield return baseDirectory + value;
    }
}