[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.
addressbook
basket
blog
book
calendar
comment
company
email
file
folder
i18ncontent
keyword
link
mailinglist
person
tag
timeseries
vcsfile