title: Flask send_file return empty or blank file
body: <p>I want to return and get a .xlsx file from a server, and I use the <code>send_file</code> method in the flask library:</p>
  
  <pre><code> try:        
       print(filepath)
       return send_file(filepath, as_attachment=True, attachment_filename='Controle_Fiches_Appui.xlsx')
    except Exception as e:
       return str(e)
  </code></pre>
  
  <p>the filepath is good but when i try to download it from postman or a python client the return file is blank with 0 octect .. please help me</p>