Added option to update files via the UploadServlet.
You can now issue a PUT request with 'curl -XPUT -F file=@/tmp/file.txt http://localhost:8082/structr/upload/f30727f936f448788g64da3498853eac' to update
a file node in Structr with UUID f30727f936f448788g64da3498853eac with the content of the local file /tmp/file.txt