[Bug]: POST multipart/form-data with the form parameter supposed to be a json breaks the request

[dslynko]

Describe the bug

If I send a POST multipart/form-data request with the form parameter which is supposed to contain some json-formatted string, the request is broken. As the example I have created a simple ASP NET Core Web API solution from the VS 2019 template and added a single method. Here is the screen of the SwaggerUI:

Here is seen, that JSON string instead of request like:

curl -X 'POST' \
  'https://localhost:7263/WeatherForecast/Test' \
  -H 'accept: text/plain' \
  -H 'Content-Type: multipart/form-data' \
  -F 'someJson="{\"A\": 1, \"B\": 42}"'

becomes

curl -X 'POST' \
  'https://localhost:7263/WeatherForecast/Test' \
  -H 'accept: text/plain' \
  -H 'Content-Type: multipart/form-data' \
  -F 'A=1' \
  -F 'B=42'

As the result, ASP NET Core API method does not receive the parameter at all.

Expected behavior

JSON-formatted string is passed as string

curl -X 'POST' \
  'https://localhost:7263/WeatherForecast/Test' \
  -H 'accept: text/plain' \
  -H 'Content-Type: multipart/form-data' \
  -F 'someJson="{\"A\": 1, \"B\": 42}"'

Actual behavior

JSON-formatted string is split to separate properties and sent as separate form fields

curl -X 'POST' \
  'https://localhost:7263/WeatherForecast/Test' \
  -H 'accept: text/plain' \
  -H 'Content-Type: multipart/form-data' \
  -F 'A=1' \
  -F 'B=42'

Steps to reproduce

/~https://github.com/dslynko/swagger-post-json-issue

Exception(s) (if any)

No response

Swashbuckle.AspNetCore version

Latest

.NET Version

8

Anything else?

No response

[martincostello]

Might be a duplicate of #3014.

[martincostello]

I'm fairly sure this is a duplicate of #3014. If it's not, maybe it's similar to #3094, but that should be fixed in the latest release.

If you think it's the latter and not an issue with SwaggerUI and you're still having a problem, then we can re-open this and investigate.