Add attachments endpoints

[JovicaSusa]

We need to add inbox attachments endpoints.

[JovicaSusa]

@StoneFrog @Azdaroth I've set client to send base64 encoded string for attachments file, is that the right way to handle upload of the attachments? If so we need to change inbox uploader on core to use carrierwave-base64 gem.

[Azdaroth]

@JovicaSusa try uploading something on Core in dev and see if it works

[StoneFrog]

@JovicaSusa doesn't remote_file_url work :P ?

[JovicaSusa]

@Azdaroth It worked when I've added carrierwave-base64 gem to Core and changed inbox uploader to

mount_base64_uploader :file, InboxFileUploader

[JovicaSusa]

@StoneFrog Not sure what to do remote_file_url? Where that comes from 😄

[StoneFrog]

@JovicaSusa I think we should support both as we do now with photos. Basically you should be able to pass file as base64 encoded string or remote_file_url as external url.

[JovicaSusa]

@StoneFrog Ok, thanks. I'll check how photos work.