[web2py] how to pre-default an upload field type

[wwwgong]

Hi, 
I like to build a small web2py app to manage documents on my computer. e.g. 
db.define_table('docs',
    Field('filename'),
    Field('filetype'),
    Field('category'),
    Field('description'),
    Field('filepath','upload')
);

If I like to create a new document,e.g. 'test.doc', how can web2py 
pre-default an empty .doc file into "uploads" folder before I would later 
add content to file in OS.
In above table, a new record should be created with : 

filename='test', 

filetype='doc', 

category='test', 

description='test', 

filepath='test-xxxxxxxxxxxxxx.doc'


Thanks for your help

Wen 

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