Spring-Boot Post-Mapping with MultipartFile and Json Data

Question

I want to create a REST API to place Files using Spring-Boot. Together with the file I need to get some Metadata which I would like to get as JSON class. At fist I created a class for the metadata:

@Getter
@Setter
@ToString
@RequiredArgsConstructor
@Entity
@Table(name = FileData.TABLE_NAME)
public class FileData {
    public static final String TABLE_NAME = "file_data";

    private static final long serialVersionUID = 1342709296259278462L;

    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    @GenericGenerator(name = "native", strategy = "native")
    @Column(updatable = false)
    private Long id;

    @Column(updatable = false)
    private String userId;

    @Column(updatable = false, nullable = false)
    private String filename;

    private String description;
}

Then I created an API:

@RequestMapping("/files")
public interface FileApi {

    @Operation(summary = "Add a new File and return its id.", tags = {"File Upload"})
    @PostMapping(path = "/upload",
            consumes = {MediaType.MULTIPART_FORM_DATA})
    ResponseEntity<String> addFile(@RequestPart(value = "file") final MultipartFile file, @RequestBody FileData fileData, @Context final HttpServletRequest request);
}

I also implemented the method in my Controller. Now I am failing to implement a test for this method. I did following:

@Test
    public void uploadFile_success() throws Exception {
        final MockMultipartFile file = new MockMultipartFile("file", "test.zip", "text/plain", "test".getBytes());
        FileData fileData = new FileData();
        fileData.setFilename("bla.zip");
        fileData.setDescription("A very nice File");
        String address = "/files/upload";
        mockMvc.perform(MockMvcRequestBuilders.multipart(address)
                        .file(file)
                        .content(MAPPER.writeValueAsString(fileData))
                        .contentType(MediaType.MULTIPART_FORM_DATA))
                .andExpect(status().isOK())
                .andExpect(content().string("OK"))
                .andReturn();
    }

When I run the test, I get HTTP error code 415 and the following error message:

Content type 'multipart/form-data' not supported

Regarding Swagger UI the Request Body shall have multipart/fomr-data. I also removed the contentType call (with same result) and changed it to "application/json" (same result, but with this content type).

Can andybody tell me what content type I have to set?

I could run the test with contentType "application/json" by removing the consumes entry in PostMapping, but in Swagger UI I have then to place the data of the file in a string field in JSON data and cannot transfer a binary file. I think this is not the right way to do it...