XLS file is generated in onChange event. Binary data gets saved in the field, and can be downloaded and opened without a problem. Only problem is that filename offered by the browser's save dialog is modelName_modelID (until model gets saved).

Snippet from _view.xml

 <field name="node_candidates_fn" invisible="1"/>
 <field name="node_candidates" filename="node_candidates_fn" readonly="True" nolabel="0"/>

node_candidates_fn is set in _defaults using:

'node_candidates_fn': 'NodeCandidatesFromDefault.xls',

And when writing a binary file using:

to_save = {'node_candidates': base64_data,  
           'node_candidates_fn': "NodeCandidatesFromSaveToBinary.xls"}
self.write(cr, uid, ids[0], to_save)

To be on the safe side, result from the onChange event (after saving to binary field) is:

 return {'value': {'node_candidates': base64_data,
                   'node_candidates_fn': 'NodeCandidatesFromOnChangeRetVal.xls'}}

When XLS file gets generated, the download widget shows: "Download NodeCandidatesFromSaveToBinary.xls", but when I click on the link, the filename that is offered by the browser's save dialog is modelName_modelId.

Note: If I save the model, filename offered by the browser's save dialog is NodeCandidatesFromSaveToBinary.xls. Saving model is not preferred!