initial multipart implementation

[jjl]

Please do not merge, I haven't actually written the tests for this yet, merely verified the existing ones do not break.

Aside from this, is this generally acceptable? Is it worth me putting in the work on the tests to make this work?

[swelham]

This looks good to me so far, many thanks!

The only thing I would like to suggest is changing the for statement usage to use Enum.map. I feel this would be more concise and consistent with other areas of the codebase.