files are identified of their type firstly by file extension and secondly, for xml files, by the schema they specify; If a file has no schema speicifed upon upload but is valid for the type that is specified, the schema information is added; at each point in time of the file's living time in the system the schema info has to stay; hence upon edit it is reinserted if deleted by user; edit validation has been fixed (currently only xml files can be edited) to using this information;