I'm trying to get a code to allow users to upload images in some website module, that is the solution:

And with that code you can upload a file to ir.attachment.
 

-------------------------------xml------------------------
        <form t-attf-action="/someurl" method="post" role="form" nctype="multipart/form-data"> 
                <span class="btn btn-primary btn-file mt16">
		    Upload picture 
		    <input type="file" name="picture"/>
		</span>
        </form>                
------------------------------------------------------------


---------------------------controller-----------------------

@http.route('/someurl', type='http', auth="user", methods=['POST'], website=True)

def image_handle(self, **post): 
        post_file = []  # List of file to add to ir_attachment once we have the ID
        post_description = []  # Info to add after the message
        values = {}

        for field_name, field_value in post.items():
            if hasattr(field_value, 'filename'):
                post_file.append(field_value)

        Forum = request.registry['forum.forum']
        for field_value in post_file:
	   attachment_value = {
	       'name': field_value.filename,
	       'res_name': field_value.filename,
               'res_model': 'forum.forum',
               'res_id': forum.id,
	       'datas': base64.encodestring(field_value.read()),
	       'datas_fname': field_value.filename,
	   }
	   request.registry['ir.attachment'].create(request.cr, SUPERUSER_ID, attachment_value, context=request.context) 

Many thanks for your collaboration!