fix: body with ContentTypeFilter should update OpenAPI

huma.go

@@ -804,11 +804,17 @@ func Register[I, O any](api API, op Operation, handler func(context.Context, *I)
 			if op.Responses[statusStr].Content == nil {
 				op.Responses[statusStr].Content = map[string]*MediaType{}
 			}
+			// Check if the field's type implements ContentTypeFilter
+			contentType := "application/json"
+			if reflect.PointerTo(f.Type).Implements(reflect.TypeFor[ContentTypeFilter]()) {
+				instance := reflect.New(f.Type).Interface().(ContentTypeFilter)
+				contentType = instance.ContentType(contentType)
+			}
 			if len(op.Responses[statusStr].Content) == 0 {
-				op.Responses[statusStr].Content["application/json"] = &MediaType{}
+				op.Responses[statusStr].Content[contentType] = &MediaType{}
 			}
-			if op.Responses[statusStr].Content["application/json"] != nil && op.Responses[statusStr].Content["application/json"].Schema == nil {
-				op.Responses[statusStr].Content["application/json"].Schema = outSchema
+			if op.Responses[statusStr].Content[contentType] != nil && op.Responses[statusStr].Content[contentType].Schema == nil {
+				op.Responses[statusStr].Content[contentType].Schema = outSchema
 			}
 		}
 	}

huma_test.go

@@ -2005,6 +2005,31 @@ func TestOpenAPI(t *testing.T) {
 	})
 }
 
+type CTFilterBody struct {
+	Field string `json:"field"`
+}
+
+func (b *CTFilterBody) ContentType(ct string) string {
+	return "application/custom+json"
+}
+
+var _ huma.ContentTypeFilter = (*CTFilterBody)(nil)
+
+func TestContentTypeFilter(t *testing.T) {
+	_, api := humatest.New(t, huma.DefaultConfig("Test API", "1.0.0"))
+	huma.Get(api, "/ct-filter", func(ctx context.Context, i *struct{}) (*struct {
+		Body CTFilterBody
+	}, error) {
+		return nil, nil
+	})
+
+	responses := api.OpenAPI().Paths["/ct-filter"].Get.Responses["200"].Content
+	assert.Equal(t, 1, len(responses))
+	for k := range responses {
+		assert.Equal(t, "application/custom+json", k)
+	}
+}
+
 type IntNot3 int
 
 func (i IntNot3) Resolve(ctx huma.Context, prefix *huma.PathBuffer) []error {