[etwist] fix handling of multiple files per field html5 permits multiple files uploads, which can be expressed as:: <input type='file' multiple='multiple' /> This changeset avoids previous crash. Nothing is changed when a single file is uploaded (backward compat is thus preserved). When multiple files are uploaded for a single html input tag, the corresponding web request form key receives a list of tuples like [('filename-1', IStream1), ('filename-2', IStream2), ...]. closes #2847207.

fontcolor = 'black'
bgcolor = '#FFFFFF'
stylesheets = sheet['stylesheets'] + ['%s/mycube.css' % datadir_url]