File Format Change from csv to string while adding data in request body

I am adding one parameter in request body if POST request (Content-Type= "multipart-form/data") but after adding when i set edited body in kong request, file format changed from csv to string. And i am getting 400 Bad Request because MultiPartFile parameter not found in Request Body. Pls help

Code:

local body = kong.request.get_body()
body["vendor"]="{ "vendorId" : "vikas", "vendorName" : "vikas" }"
kong.service.request.set_body(body)

Request Body:

Reproduce the multipart request using kong.request.get_body() and kong.service.request.set_body() with the shown vendor value, then inspect the request-body handling entry points for how multipart file fields are represented. Done means the edited request preserves the multipart file parameter and no longer produces the reported 400 Bad Request.