Add support to attachments plugin

[gabisurita]

I'm not sure if supporting plugins is in the scope of the Python client, but I think it would be useful to have attachments support.

My idea is to do this through an argument "files" on create, update and patch record methods.

It would also be good to raise an appropriate exception message if the server doesn't support the plugin.

[Natim]

Yes it would be great. Not that we added the support for kinto-http.js

[leplatrem]

Why not!

Maybe we should start by draft some dream code here to see how it would like.

For info, we had some python code here: https://github.com/Kinto/kinto-attachment/blob/master/scripts/upload.py

[leplatrem]

Sample code: https://gist.github.com/leplatrem/98b0ff1818053232082b8551b26c4a0d