[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.

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<div id="ajaxroot">

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  <div class="ajaxHtmlHead">

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    <script src="http://foo.js" type="text/javascript"> </script>

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    <link rel="stylesheet" type="text/css" media="all" href="qunit.css" />

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  </div>

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  <h1>Hello</h1>

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</div>