title: How to handle file upload validations using Flask-Marshmallow?
body: <p>I'm working with <code>Flask-Marshmallow</code> for validating <code>request</code> and <code>response</code> schemas in <code>Flask app</code>. I was able to do simple validations for <code>request.form</code> and <code>request.args</code> when there are simple fields like <code>Int</code>, <code>Str</code>, <code>Float</code> etc. </p>
  
  <p>I have a case where I need to upload a file using a form field - <code>file_field</code>. It should contain the file content.</p>
  
  <p>How can I validate if this field is present or not and what is the format of file etc.</p>
  
  <p>Is there any such field in <code>Marshmallow</code> that I can use like <code>fields.Int()</code> or <code>fields.Str()</code></p>
  
  <p>I have gone through the documentation <a href="https://marshmallow.readthedocs.io/en/latest/api_reference.html#marshmallow.fields" rel="noreferrer">here</a> but haven't found any such field.</p>