title: Request with multipart/form-data returns 415 error
body: <p>I need to receive this request using <code>Spring</code>:</p>
  
  <pre><code>POST /test HTTP/1.1
  user-agent: Dart/2.8 (dart:io)
  content-type: multipart/form-data; boundary=--dio-boundary-3791459749
  accept-encoding: gzip
  content-length: 151
  host: 192.168.0.107:8443
  
  ----dio-boundary-3791459749
  content-disposition: form-data; name="MyModel"
  
  {"testString":"hello world"}
  ----dio-boundary-3791459749--
  </code></pre>
  
  <p>But unfortunately this <code>Spring</code> endpoint:</p>
  
  <pre><code>@PostMapping(value = "/test", consumes = MediaType.APPLICATION_OCTET_STREAM_VALUE)
  public void test(@Valid @RequestPart(value = "MyModel") MyModel myModel) {
      String testString = myModel.getTestString();
  }
  </code></pre>
  
  <p>returns <code>415</code> error:</p>
  
  <pre><code>Content type 'multipart/form-data;boundary=--dio-boundary-2534440849' not supported
  </code></pre>
  
  <p>to the client.</p>
  
  <p>And this(same endpoint but with the <code>consumes = MULTIPART_FORM_DATA_VALUE</code>):</p>
  
  <pre><code>@PostMapping(value = "/test", consumes = MediaType.MULTIPART_FORM_DATA_VALUE)
  public void test(@Valid @RequestPart(value = "MyModel") MyModel myModel) {
      String testString = myModel.getTestString();
  }
  </code></pre>
  
  <p>again returns <code>415</code> but, with this message:</p>
  
  <pre><code>Content type 'application/octet-stream' not supported
  </code></pre>
  
  <p>I already successfully used this endpoint(even without <code>consumes</code>) with this old request:</p>
  
  <pre><code>POST /test HTTP/1.1
  Content-Type: multipart/form-data; boundary=62b81b81-05b1-4287-971b-c32ffa990559
  Content-Length: 275
  Host: 192.168.0.107:8443
  Connection: Keep-Alive
  Accept-Encoding: gzip
  User-Agent: okhttp/3.8.0
  
  --62b81b81-05b1-4287-971b-c32ffa990559
  Content-Disposition: form-data; name="MyModel"
  Content-Transfer-Encoding: binary
  Content-Type: application/json; charset=UTF-8
  Content-Length: 35
  
  {"testString":"hello world"}
  --62b81b81-05b1-4287-971b-c32ffa990559--
  </code></pre>
  
  <p>But unfortunately now I need to use the first described request and I can't add additional fields to it.</p>
  
  <p>So, I need to change the <code>Spring</code> endpoint, but how?</p>