Multipart data loader archive/extractor format?

[Fil]

In addition to tar and zip, we could have multipart archives.

After checking IANA#multipart and IANA#message, it looks like the most viable format would be multipart/form-data.

It has no IANA File extension associated :( — however we could adopt {archive}.multipart.{js,sh…} internally.

For JavaScript data loaders we'll need to write (or import) an encoder. And we'll need to write (or import) a decoder. "npm:form-data-encoder" looks like a good encoder.

Refs: #1222, #207.