http://docs.pylonsproject.org/projects/colander/en/latest/binding.html

Here's an (untested and incomplete) example:

  @colander.deferred
  def deferred_upload_widget(field, bindargs):
      request = bindargs['request']
      session = request.session
      tmpstore = MyTmpStore(session)
      return deform.widget.FileUploadWidget(tmpstore)

  class MyUploadSchema(colander.Schema):
      upload = colander.SchemaNode(deform.FileData(),
                                   widget=deferred_upload_widget)

  upload_schema = MyUploadSchema()

then, in your view logic (where you have access to the request object):

  schema = upload_schema.bind(request=request)
  form = Form(schema, ...

Jeff