This exception is a trick, to stop adding files to the file list, when too
many files don't match the allowed extension (see
wjhk.jupload2.gui.FilePanelDataModel2#addFile(File, File)), or any
other control of the current upload policy (see
UploadPolicy.createFileData(File, File).
This class should be used for all implementations of FileData or UploadPolicy
that want to throw an IO exception, and need to be conform with the interface
definition.