Incorrect generation of Retrofit interfaces for file uploads

Bug Report Checklist

[x] Have you provided a full/minimal spec to reproduce the issue?
[x] Have you validated the input using an OpenAPI validator (example)?
[ ] Have you tested with the latest master to confirm the issue still exists?
[x] Have you searched for related issues/PRs?
[ ] What's the actual output vs expected output?
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Description

I have a project using the Kotlin generator and jvm-retrofit2 as the library. For file uploads, my OpenAPI schema has the following request body definition:

      requestBody:
        content:
          application/octet-stream:
            schema:
              format: binary
              type: string
        required: true

The resulting interface uses java.io.File as the body for the Retrofit interface. However, Retrofit doesn't support File for file uploads out of the box, resulting in an exception at runtime.

openapi-generator version

7.8.0

OpenAPI declaration file content or url

https://github.com/fyreplace/fyreplace-android/blob/develop/app/src/main/assets/openapi.yaml

Generation Details

The code is generated using the official Gradle plugin.

Steps to reproduce

Clone https://github.com/fyreplace/fyreplace-android
Run ./gradlew openapiGenerate inside the repository
Check the resulting app/build/openapi/src/main/kotlin/app/fyreplace/api/UsersEndpointApi.kt, and look for setCurrentUserAvatar()

Related issues/PRs

Suggest a fix

A custom Converter for files can be created to handle files in request bodies. I have managed to make my uploads work with the following code:

import okhttp3.RequestBody
import okhttp3.RequestBody.Companion.asRequestBody
import retrofit2.Converter
import retrofit2.Retrofit
import java.io.File
import java.lang.reflect.Type

class FileConverterFactory : Converter.Factory() {
    override fun requestBodyConverter(
        type: Type,
        parameterAnnotations: Array<out Annotation>,
        methodAnnotations: Array<out Annotation>,
        retrofit: Retrofit
    ) = when (type) {
        File::class.java -> Converter<File, RequestBody> { it.asRequestBody(null) }
        else -> null
    }

    companion object {
        fun create() = FileConverterFactory()
    }
}

The equivalent Java implementation would look something like this:

import androidx.annotation.Nullable;
import java.io.File;
import java.lang.annotation.Annotation;
import java.lang.reflect.Type;
import okhttp3.RequestBody;
import retrofit2.Converter;
import retrofit2.Retrofit;

public class FileConverterFactory extends Converter.Factory {
    @Nullable
    @Override
    public Converter<?, RequestBody> requestBodyConverter(Type type, Annotation[] parameterAnnotations, Annotation[] methodAnnotations, Retrofit retrofit) {
        return type == File.class
                ? (File file) -> RequestBody.create(file, null)
                : null;
    }

    public static FileConverterFactory create() {
        return new FileConverterFactory();
    }
}

Adding an instance of this FileConverterFactory to the list of converter factories when creating the API client makes Retrofit perfectly happy to send File objects in its requests.

OpenAPITools / openapi-generator