title: Get form data in handleSubmit
body: <p>Can anyone help me?</p>
  
  <p>In the Admin-on-rest framework, I want to write a handleSubmit in SimpleForm. How to get form data in handleSubmit.</p>
  
  <p>I have two file input: <code>FileInput</code> and <code>ImageInput</code> and one <code>TextInput</code>. I want to send files and text to the server after submitting the form. The input files use the react-dropzone and don't send the files to the server after submitting.</p>
  
  <p>I can not find any example for this.</p>
  
  <pre><code>let filex;    
  &lt;SimpleForm handleSubmit={(data)=&gt;{ console.log(data); }}&gt;
           &lt;TextInput label="عنوان کتاب" source="title" validate={required} /&gt;
              &lt;RichTextInput label="توضیحات" source="description" toolbar={[ 
                  ['bold', 'italic', 'underline', 'link'],
                  [{ 'direction': 'rtl' }],
                  [{ 'align': [] }],
                  ['clean'],
                  [{ 'list': 'ordered'}, { 'list': 'bullet' }],
              ]} validate={required} /&gt;
              &lt;ImageInput source="bookImg" 
              label="تصویر کتاب" 
              input={{
                  value: filex,
                  onChange: (file) =&gt; {
                      filex = file;
                  }
              }}
              validate={required} accept="image/*"&gt;
                  &lt;ImageField source="src" title="title" /&gt;
              &lt;/ImageInput&gt;
              &lt;FileInput source="bookPdf" label="فایل کتاب" validate={required} accept="application/pdf"&gt;
                  &lt;FileField source="src" title="title" /&gt;
              &lt;/FileInput&gt;
      &lt;/SimpleForm&gt;
  </code></pre>