Add ability to specify the uploaded file content-type

[leplatrem]

If unset, let the server assign it as it does now.

If set the attachment passed to kinto.js's collection.createRecord(..) should contain the type

https://github.com/Kinto/kinto-admin/blob/c648566146ff2670471024c4b4f8f0e26939df4b/src/components/record/RecordCreate.tsx#L37-L41

• kinto.js supports extracting the type from the data URL