[web2py] help with extra_fields on auth_user needed

[Adi]

I added an extra field for the purpose of having digital signature image in 
it. The image displayed first time I added the field, but then stopped and 
now doesn't display on the  local server. At the same time, another image 
from a different table displays fine in the same view. 

Am I doing something wrong with the extra field? 

db.py
auth.settings.extra_fields['auth_user']= [
    Field('digital_signature', 'upload',label=T('Digital Signature')),
  ]

view: 

{{if po.auth_user.digital_signature:}}
    <img src="{{=URL('download', args=po.auth_user.digital_signature, 
scheme=True, host=True)}}" width="240px" height="60px" /><br/>
    {{=po.auth_user.first_name}} {{=po.auth_user.last_name}}
{{else:}}
    ____________________________________
    {{pass}}
    
, Date: {{=request.now.date().strftime("%A, %B %d, %Y")}}