fix pb with bytes field processing: currently when an existing file is edited and no new file is specified, it finds no value in req.form and so try to detach the current file, which is wrong. In that case, nothing should be done. So introduce a new UnmodifiedField exception that may be raised in field's process_form_value method (catched in field.process_posted).

Document: cubicweb-doc
Title: CubicWeb documentation
Author: Logilab
Abstract: Some base documentation for CubicWeb users and developpers
Section: Apps/Programming

Format: HTML
Index: /usr/share/doc/cubicweb-documentation/index.html
Files: /usr/share/doc/cubicweb-documentation/*.html