Serializer for multipart/form-data

[GraphZal]

I am currently writing function to validate data from a .xlsx file. This validation creates a simple list containing the rules violated, as well as another excel file with a detailed report that shows which rows break which rules and why.
I'd like to turn that function into an API endpoint, so my users can trigger the validation themselves. The API endpoint should return both, the simple list (probably as a json object), as well as the detailed report file.
In short, I'd like to return a multipart/form-data response from my API endpoint, which does not yet seem to be supported in Plumber.

Other people also seem to have need for something similar, see e.g. Return JSON and RData from an endpoint or Plumber returning multiple images in one response?.

Therefore, I'd like to request the addition of a multipart/form-data serializer to Plumber.

[meztez]

@GraphZal The simplest way is to return them in a single zip archive. The browser does not know what to do with multipart/form-data response.

[meztez]

Note, as_attachment, support multi files or object, return zip?

[meztez]

https://github.com/rstudio/plumber/pull/922/files