Set the FileFilter property of the URL Control to a string containing a comma delimited list of the desired file extensions. Do not use a period before each extension . . .
In code: ctlURL.FileFilter = "jpg, gif"
You can also set this property in your user control's html markup. The property is one of the few that is persisted in Viewstate so does not have to be set each time the page/control loads.
Bill, WESNet Designs