Add base64-encoded images support

[yunmikun2]

It is common case with json-api to send an image as base64-encoded string. Here goes support for this functionality.

Now when casting an image in base64-encoded format, it will be casted to a
binary image with generated file name.

[pironim]

I looking for something similar and maybe it's worth to have special key like Arc
something like that %{filename: filename, base64: base64}

stolen from https://github.com/stavro/arc/pull/108/files

[achempion]

I have exactly the same case in one of the projects and I've resolved it by handeling API request data by decoding base64 encoded file before passing it to the Waffle. Probably, we can extend documentation about how to do it, but I'm not yet sure about should we make it as a scope of waffle_ecto library. Feedback is welcome.

[yunmikun2]

@achempion, well, the first way of handling this case I can think of, is to define an ecto schema that parses params before sending it into Ecto.Changeset.cast/4, which seems like overdo and, moreover, drags a lot of boilerplate.

If you ask me about a proper solution, this should be handled in the ecto-type itself (Waffle.Ecto.Type) instead of Waffle.Ecto.Schema.cast_attachments/4 (which is a macro for, I guess, some obscure optimization reasons), and the definition of Waffle.Ecto.Schema.cast_attachments/4 should be replaced with defdelegate to Ecto.Changeset.cast/4 (only for compatibility reasons).

[yunmikun2]

Any info on whether it's going to be merged?

[szsoppa]

Hey guys, are there any plans to merge this PR?