Returns or sets the default file extension in the ShowFileDialog dialog box.
Image Admin control
object.DefaultExt[=extension]
String
The extension of the currently selected filter overrides the default extension.
The default extension is used only when there is no extension associated with the filter (for example, 'All files *.*').
The first three characters of the string you specify are used as the file extension when the user does not specify a file extension.
ShowFileDialog method