Use DBID in place of FILE in the pseudoRef to DHForm

The idea to use "POOL object placement" as a substitute to an actual object Reference turned out to now work in case somebody generates several files with the same name and then tries to merge them. (Placement contains the file name).
This change replaces the file name with the unique POOL DBID generated when creating a file to fix the problem.
The fix is for single DataHeaderForm writing mode not used in production.