Adding info on multipart/form-data to readme

Author: Orbifoldt

README.md

@@ -276,6 +276,69 @@ org.acme.openapi.simple.api.DefaultApi/byeGet/CircuitBreaker/failureRatio=3.14
 org.acme.openapi.simple.api.DefaultApi/byeGet/CircuitBreaker/successThreshold=22
 ````
 
+## Multipart/form-data input
+The rest client also supports request with mime-type multipart/form-data and, if the schema of the request body is known in advance, we can also automatically generate the models of the request bodies. 
+
+You need to add the following additional dependency to your `pom.xml`:
+```xml
+<dependency>
+  <groupId>io.quarkus</groupId>
+  <artifactId>quarkus-resteasy-multipart</artifactId>
+</dependency>
+```
+For any multipart/form-data operation a model for the request body will be generated. Each part of the multipart is a field in this model that is annotated with the following annotations:
+- `javax.ws.rs.FormParam`, where the value parameter denotes the part name, 
+- `org.jboss.resteasy.annotations.providers.multipart.PartType`, where the parameter is the jax-rs MediaType of the part (see below for details),
+- and, if the part contains a file, `org.jboss.resteasy.annotations.providers.multipart.PartFilename`, with a default generate value parameter that will be passed as the fileName sub-header in the Content-Disposition header of the part.
+
+For example, the model for a request that requires a file, a string and some complex object will look like this: 
+```java
+public class MultipartBody {
+
+    @FormParam("file")
+    @PartType(MediaType.APPLICATION_OCTET_STREAM)
+    @PartFilename("defaultFileName")
+    public File file;
+
+    @FormParam("fileName")
+    @PartType(MediaType.TEXT_PLAIN)
+    public String fileName;
+
+    @FormParam("someObject")
+    @PartType(MediaType.APPLICATION_JSON)
+    public MyComplexObject someObject;
+}
+```
+
+Then in the client the `org.jboss.resteasy.annotations.providers.multipart.MultipartForm` annotation is added in front of the multipart parameter:
+```java
+@Path("/echo")
+@RegisterRestClient
+public interface MultipartService {
+    
+    @POST
+    @Consumes(MediaType.MULTIPART_FORM_DATA)
+    @Produces(MediaType.TEXT_PLAIN)
+    String sendMultipartData(@MultipartForm MultipartBody data);
+    
+}
+```
+See [Quarkus - Using the REST Client with Multipart](https://quarkus.io/guides/rest-client-multipart) and the [RESTEasy JAX-RS specifications](https://docs.jboss.org/resteasy/docs/4.7.5.Final/userguide/html_single/index.html) for more details.
+
+Importantly, if some multipart request bodies contain complex objects (i.e. non-primitives) you need to explicitly tell the Open API generator to create models for these objects by setting the `skip-form-model` property corresponding to your spec in the `application.properties` to `false`, e.g.:
+```properties
+quarkus.openapi-generator.codegen.spec."my-multipart-requests.yml".skip-form-model=false
+```
+
+### Default content-types according to OpenAPI Specification and limitations
+The [OAS 3.0](https://github.com/OAI/OpenAPI-Specification/blob/main/versions/3.0.3.md#special-considerations-for-multipart-content) specifies the following default content-types for a multipart:
+- If the property is a primitive, or an array of primitive values, the default Content-Type is `text/plain`
+- If the property is complex, or an array of complex values, the default Content-Type is `application/json`
+- If the property is a `type: string` with `format: binary` or `format: base64` (aka a file object), the default Content-Type is `application/octet-stream`
+
+A different content-type may be defined in your api spec, but this is not yet supported in the code generation. Also, this "annotation-oriented" approach of RestEasy (i.e. using `@MultipartForm` to denote the multipart body parameter) does not seem to properly support the unmarshalling of arrays of the same type (e.g. array of files), in these cases it uses Content-Type equal to `application/json`.
+
+
 ## Generating files via InputStream
 
 Having the files in the `src/main/openapi` directory will generate the REST stubs by default. Alternatively, you can implement the `io.quarkiverse.openapi.generator.codegen.OpenApiSpecInputProvider`

integration-tests/src/main/resources/application.properties

@@ -24,5 +24,4 @@ org.acme.openapi.simple.api.DefaultApi/byeGet/CircuitBreaker/successThreshold =
 
 quarkus.openapi-generator.codegen.spec."multipart-requests.yml".base-package=org.acme.openapi.multipart
 # By default the openapi-generator doesn't generate models for multipart requests
-quarkus.openapi-generator.codegen.spec."multipart-requests.yml".skip-form-model=false
-quarkus.log.category."org.apache.http".level=DEBUG
+quarkus.openapi-generator.codegen.spec."multipart-requests.yml".skip-form-model=false