Giving @Multipart more flexibility to set the type of Multipart

retrofit/src/main/java/retrofit2/RequestBuilder.java

@@ -72,7 +72,8 @@ final class RequestBuilder {
       @Nullable MediaType contentType,
       boolean hasBody,
       boolean isFormEncoded,
-      boolean isMultipart) {
+      boolean isMultipart,
+      String multipartType) {
     this.method = method;
     this.baseUrl = baseUrl;
     this.relativeUrl = relativeUrl;
@@ -92,7 +93,7 @@ final class RequestBuilder {
     } else if (isMultipart) {
       // Will be set to 'body' in 'build'.
       multipartBuilder = new MultipartBody.Builder();
-      multipartBuilder.setType(MultipartBody.FORM);
+      multipartBuilder.setType(MediaType.get(multipartType));
     }
   }
 

retrofit/src/main/java/retrofit2/RequestFactory.java

@@ -76,6 +76,7 @@ static RequestFactory parseAnnotations(Retrofit retrofit, Method method) {
   private final boolean hasBody;
   private final boolean isFormEncoded;
   private final boolean isMultipart;
+  private final String multipartType;
   private final ParameterHandler<?>[] parameterHandlers;
   final boolean isKotlinSuspendFunction;
 
@@ -89,6 +90,7 @@ static RequestFactory parseAnnotations(Retrofit retrofit, Method method) {
     hasBody = builder.hasBody;
     isFormEncoded = builder.isFormEncoded;
     isMultipart = builder.isMultipart;
+    multipartType = builder.multipartType;
     parameterHandlers = builder.parameterHandlers;
     isKotlinSuspendFunction = builder.isKotlinSuspendFunction;
   }
@@ -116,7 +118,8 @@ okhttp3.Request create(Object[] args) throws IOException {
             contentType,
             hasBody,
             isFormEncoded,
-            isMultipart);
+            isMultipart,
+            multipartType);
 
     if (isKotlinSuspendFunction) {
       // The Continuation is the last parameter and the handlers array contains null at that index.
@@ -161,6 +164,7 @@ static final class Builder {
     boolean hasBody;
     boolean isFormEncoded;
     boolean isMultipart;
+    @Nullable String multipartType;
     @Nullable String relativeUrl;
     @Nullable Headers headers;
     @Nullable MediaType contentType;
@@ -251,6 +255,7 @@ private void parseMethodAnnotation(Annotation annotation) {
           throw methodError(method, "Only one encoding annotation is allowed.");
         }
         isMultipart = true;
+        multipartType = ((Multipart) annotation).type();
       } else if (annotation instanceof FormUrlEncoded) {
         if (isMultipart) {
           throw methodError(method, "Only one encoding annotation is allowed.");

retrofit/src/main/java/retrofit2/http/Multipart.java

@@ -29,4 +29,7 @@
 @Documented
 @Target(METHOD)
 @Retention(RUNTIME)
-public @interface Multipart {}
+public @interface Multipart {
+  /** Sets the type(MediaType) on MultipartBody. When calling the */
+  String type() default "multipart/form-data";
+}

retrofit/src/test/java/retrofit2/RequestFactoryTest.java

@@ -3252,6 +3252,54 @@ Call<ResponseBody> method(@Tag List<String> one, @Tag List<Long> two) {
     }
   }
 
+  @Test
+  public void shouldBuildRequestBodyWithContentTypeSuccessfully() throws IOException {
+    class Example {
+      @Multipart(type = "multipart/form-data; charset=utf-8") //
+      @POST("/foo/bar/") //
+      Call<ResponseBody> method(@Part("ping") String ping, @Part("kit") RequestBody kit) {
+        return null;
+      }
+    }
+
+    Request request = buildRequest(Example.class, "pong", RequestBody.create(TEXT_PLAIN, "kat"));
+    assertThat(request.method()).isEqualTo("POST");
+    assertThat(request.headers().size()).isZero();
+    assertThat(request.url().toString()).isEqualTo("http://example.com/foo/bar/");
+
+    RequestBody body = request.body();
+    assertThat(body.contentType().toString())
+        .startsWith("multipart/form-data; charset=utf-8; boundary=");
+
+    Buffer buffer = new Buffer();
+    body.writeTo(buffer);
+    String bodyString = buffer.readUtf8();
+
+    assertThat(bodyString)
+        .contains("Content-Disposition: form-data;")
+        .contains("name=\"ping\"\r\n")
+        .contains("\r\npong\r\n--")
+        .contains("name=\"kit\"")
+        .contains("\r\nkat\r\n--");
+  }
+
+  @Test
+  public void shouldFailRequestBodyWithContentTypeInvalid() throws IOException {
+    class Example {
+      @Multipart(type = "invalid-type") //
+      @POST("/foo/bar/") //
+      Call<ResponseBody> method(@Part("ping") String ping, @Part("kit") RequestBody kit) {
+        return null;
+      }
+    }
+    try {
+      buildRequest(Example.class, "pong", RequestBody.create(TEXT_PLAIN, "kat"));
+      fail();
+    } catch (IllegalArgumentException e) {
+      assertThat(e).hasMessageContaining("No subtype found for: \"invalid-type\"");
+    }
+  }
+
   private static void assertBody(RequestBody body, String expected) {
     assertThat(body).isNotNull();
     Buffer buffer = new Buffer();