add the name of the currently attached file, if the file already exists (i.e in the edition form)
class ToTo(EntityType):
permissions = {
'read': ('managers',),
'add': ('managers',),
'update': ('managers',),
'delete': ('managers',),
}
attr = String()
class attr(RelationType):
permissions = {
'read': ('managers', ),
'add': ('managers', RRQLExpression('S bla Y'),),
'delete': ('managers',),
}