[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.