Very interesting question, that allow me to give some light to a couple of hidden features of Odoo that allow you to do exactly that using the report action like this:
@api.multi
def emp_system_help(self):
return {
'type': 'ir.actions.report.xml',
'report_type': 'controller',
//for v9 and v10
'report_file': '/web/content/res.partner/7/image/axel.png?download=true',
// for v7 and v8
// 'report_file': '/web/binary/saveas?model=ir.attachment&field=datas&filename_field=datas_fname&id=194'
}
Just need to return the action definition of type ir.actions.report.xml and report_type controller that will allow you to download any kind of file from any URL that you could specify in the report_file of the action dict
As you could see the format of the report_file URL contains the information referring to the res.partner model and record id=7 and the field image of that record to be downloaded and axel.png as the filename of the image file to be downloaded.
This means that you could build URLs to access the binary field values of any model in your Odoo instance. Here I left you the complete formats supported by the /web/content controller routes as states the method definition with it's default argument values
For v9 and v10
@http.route(['/web/content',
'/web/content/<string:xmlid>',
'/web/content/<string:xmlid>/<string:filename>',
'/web/content/<int:id>',
'/web/content/<int:id>/<string:filename>',
'/web/content/<int:id>-<string:unique>',
'/web/content/<int:id>-<string:unique>/<string:filename>',
'/web/content/<string:model>/<int:id>/<string:field>',
'/web/content/<string:model>/<int:id>/<string:field>/<string:filename>'], type='http', auth="public")
def content_common(self, xmlid=None, model='ir.attachment', id=None, field='datas', filename=None, filename_field='datas_fname', unique=None, mimetype=None, download=None, data=None, token=None):
Hope this helps b
Get an idea from here: http://learnopenerp.blogspot.com/2020/06/write-binary-data-nto-zip-file-and-downlaod-it-on-button-click-in-odoo.html