Custom config section containing collection

I'm having trouble getting a custom config section to work. It's some code I got from the web in an effort to try to understand this area a little better and enable me to get to where I want to ultimatly be, my own custom config section.

The error I get when I run the code in a console app is ' Unrecognized attribute 'extension'. Note that attribute names are case-sensitive.'

The code in the main app to get things going is

var conf = ConfigurationManager.GetSection("uploadDirector");

and this is where the exception appears.

This is the config section I am hoping/trying to achieve

<AuthorisedClients>
    <AuthorisedClient name="Client">
      <Queue id="1" />
      <Queue id="7" />
    </AuthorisedClient>
    <AuthorisedClient name="Client2">
      <Queue id="3" />
      <Queue id="4" />
    </AuthorisedClient>
  </AuthorisedClients>

Here's the code I have got from the web

.config file

<uploadDirector>
    <filegroup name="documents" defaultDirectory="/documents/">
      <clear/>
      <add extension="pdf" mime="application/pdf" maxsize="100"/>
      <add extension="doc" mime="application/word" maxsize="500"/>
    </filegroup>
    <filegroup name="images">
      <clear/>
      <add extension="gif" mime="image/gif" maxsize="100"/>
    </filegroup>
  </uploadDirector>

UploadDirectorConfigSection.cs

public class UploadDirectorConfigSection : ConfigurationSection {

        private string _rootPath;

        public UploadDirectorConfigSection() {

        }

        [ConfigurationProperty("rootpath", DefaultValue="/", IsRequired=false, IsKey=false)]
        [StringValidator(InvalidCharacters=@"~!.@#$%^&*()[]{};'\|\\")]
        public string RootPath {
            get { return _rootPath; }
            set { _rootPath = value; }
        }

        [ConfigurationProperty("", IsRequired = true, IsKey = false, IsDefaultCollection = true)]
        public FileGroupCollection FileGroups {
            get { return (FileGroupCollection) base[""]; }
            set { base[""] = value; }
        }
    }

FileGroupCollection.cs

public class FileGroupCollection : ConfigurationElementCollection {
        protected override ConfigurationElement CreateNewElement() {
            return new FileGroupElement();
        }

        protected override object GetElementKey(ConfigurationElement element) {
            return ((FileGroupElement) element).Name;
        }

        public override ConfigurationElementCollectionType CollectionType {
            get {
                return ConfigurationElementCollectionType.BasicMap;
            }
        }

        protected override string ElementName {
            get {
                return "filegroup";
            }
        }

        protected override bool IsElementName(string elementName) {
            if (string.IsNullOrWhiteSpace(elementName) || elementName != "filegroup")
                return false;
            return true;
        }

        public FileGroupElement this[int index] {
            get { return (FileGroupElement) BaseGet(index); }
            set {
                if(BaseGet(index) != null)
                    BaseRemoveAt(index);
                BaseAdd(index, value);
            }
        }
    }

FileGroupElement.cs

public class FileGroupElement : ConfigurationElement {

        [ConfigurationProperty("name", IsKey=true, IsRequired = true)]
        [StringValidator(InvalidCharacters = @" ~.!@#$%^&*()[]{}/;'""|\")]
        public string Name {
            get { return (string) base["name"]; }
            set { base["name"] = value; }
        }

        [ConfigurationProperty("defaultDirectory", DefaultValue = ".")]
        public string DefaultDirectory {
            get { return (string) base["Path"]; }
            set { base["Path"] = value; }
        }

        [ConfigurationProperty("", IsDefaultCollection = true, IsRequired = true)]
        public FileInfoCollection Files {
            get { return (FileInfoCollection) base[""]; }
        }
    }

FileInfoCollection.cs

public class FileInfoCollection : ConfigurationElementCollection {
        protected override ConfigurationElement CreateNewElement() {
            return new FileInfoCollection();
        }

        protected override object GetElementKey(ConfigurationElement element) {
            return ((FileInfoElement) element).Extension;
        }
    }

FileInfoElement.cs

public class FileInfoElement : ConfigurationElement {

        public FileInfoElement() {
            Extension = "txt";
            Mime = "text/plain";
            MaxSize = 0;
        }

        [ConfigurationProperty("extension", IsKey = true, IsRequired = true)]
        public string Extension {
            get { return (string)base["extension"]; }
            set { base["extension"] = value; }
        }

        [ConfigurationProperty("mime", DefaultValue = "text/plain")]
        public string Mime {
            get { return (string) base["mime"]; }
            set { base["mime"] = value; }
        }

        [ConfigurationProperty("maxsize", DefaultValue = 0)]
        public int MaxSize {
            get { return (int) base["maxsize"]; }
            set { base["maxsize"] = value; }
        }
    }