Hard to say. I don't know if this helps but here's how appadmin does it for a csv file:
def csv():
import gluon.contenttype
response.headers['Content-Type'] = \
gluon.contenttype.contenttype('.csv')
db = get_database(request)
query = get_query(request)
if not query:
return None
response.headers['Content-disposition'] = 'attachment;
filename=%s_%s.csv'\
% tuple(request.vars.query.split('.')[:2])
return str(db(query).select())
