[#3][m]: new check api method to check a file and extract fields.

None of this is really perfect yet.
[#4,upload][m]: working authorization method using new permissions api.
[#4,upload][m]: working (and semi-tested) authorized upload.
[#4,upload][xs]: do not try to serialize something we can't.
[#4,upload][s]: add support for region (needed as our bucket is in eu-west).