title: Cannot convert String to MultipartFile
body: <p>Please help me to resolve my problem! 
  I have bootstrap fileinput, files are included:</p>
  
  <pre><code>    &lt;link href="/assets/css/fileinput/fileinput.css" rel="stylesheet" type="text/css"&gt;
  &lt;link href="/assets/css/fileinput/fileinput-rtl.css" rel="stylesheet" type="text/css"&gt;
  &lt;script type="text/javascript" src="/assets/js/fileinput/fileinput.js"&gt;&lt;/script&gt;
  &lt;script type="text/javascript" src="/assets/js/plugins/fileinput/piexif.js"&gt;&lt;/script&gt;
  &lt;script type="text/javascript" src="/assets/js/plugins/fileinput/sortable.js"&gt;&lt;/script&gt;
  &lt;script type="text/javascript" src="/assets/js/plugins/fileinput/purify.js"&gt;&lt;/script&gt;
  </code></pre>
  
  <p>Also I have a macros for fileInput:</p>
  
  <pre><code>&lt;#macro fileInputBind path required=false fileMask=""&gt;
      &lt;@spring.bind path/&gt;
  &lt;div class="form-group"&gt;
      &lt;@label path required/&gt;
      &lt;#assign replacedPath = path?replace(".", "-") /&gt;
      &lt;#assign fileInputId = "${replacedPath}" /&gt;
      &lt;div class="col-md-12"&gt;
          &lt;input id="${fileInputId}" name="${spring.status.expression}" type="file" class="form-control file-styled"
                 accept="${fileMask}"&gt;
      &lt;/div&gt;
  &lt;/div&gt;
  &lt;/#macro&gt;
  </code></pre>
  
  <p>Here is my ftl element: </p>
  
  <pre><code>&lt;@form.fileInputBind "incidentRovdCreateForm.attachment"/&gt;
  </code></pre>
  
  <p>And variable in my form to handle this field:</p>
  
  <pre><code>private MultipartFile attachment;
  </code></pre>
  
  <p>But when @PostMapping executed I got an error in this part:</p>
  
  <pre><code>if (bindingResult.hasErrors())
              return new ModelAndView("incident-rovd/create")
                      .addObject("typeIncident1", incidentTypeLevel1Repository.findAll())
                      .addObject("typeIncident2", form.getIncidentTypeLevel1() == null ? Collections.emptyList() : incidentTypeLevel2Repository.findAllByParent(form.getIncidentTypeLevel1()))
                      .addObject("typeIncident3", form.getIncidentTypeLevel2() == null ? Collections.emptyList() : incidentTypeLevel3Repository.findAllByParent(form.getIncidentTypeLevel2()))
                      .addObject("incidentRovdCreateForm", form);
  </code></pre>
  
  <p>Error: 
  Failed to convert property value of type 'java.lang.String' to required type 'org.springframework.web.multipart.MultipartFile' for property 'attachment'; nested exception is java.lang.IllegalStateException: Cannot convert value of type 'java.lang.String' to required type 'org.springframework.web.multipart.MultipartFile' for property 'attachment': no matching editors or conversion strategy found.</p>
  
  <p>What problem can i have?</p>