Optional `file_upload` field with `with_param`s validation

[secjoa]

Description:

I'm using the with_params validation system extensively, but I'm having trouble declaring a file_upload field as optional.

Lapis provides a types.file_upload type, and I expected to be able to combine it with types.empty to allow optional uploads. However, validation fails whenever no file is submitted in the form.

local lapis = require "lapis"
local capture_errors_json = require("lapis.application").capture_errors_json
local with_params = require("lapis.validate").with_params

local app = lapis.Application()

app:post("/send_file", capture_errors_json(with_params({
  {"file", types.empty + types.file_upload}
}, function(self, params)
  print("ok")
end)))

return app

In this case, validation does not pass if the file field is not present or empty.

Expected behavior:

The validation should pass when:

• No file is uploaded (empty field), OR
• A valid file upload is provided

Actual behavior:

Validation fails when the file field is empty or not included in the request.

Questions:

What is the correct way to declare an optional file_upload field using with_params?
Is there a recommended pattern or workaround for this use case?

Additional context:

Reference to file_upload type implementation:

lapis/lapis/validate/types.moon

Lines 386 to 389 in ff773e8

	file_upload = types.partial({
	filename: types.string * -empty
	content: -types.literal("")
	})\describe "file upload"

Thanks in advance!