Not sending attachments since v5.2, and v5.4 hangs. Using Multifile Upload

What if you deactivate the “Multifile Upload Field for Contact Form 7” plugin?

I just tried it and Choose Files button is replaced by an error message. FWIW here’s the code in the Form tab:

<div class="row">
<div class="col-md-6 col-sm-5 no-padding"> Type <span class="vlabel">(required)</span></div>
<div class="col-md-6 col-sm-7"> [radio type id:request_type default:1  "Feature Request" "Bug Report"]</div>
</div>

<div class="row">
<div class="col-md-6 col-sm-5 no-padding"> Operating System <span class="vlabel">(required)</span></div>
<div class="col-md-6 col-sm-7"> [select* operatingsystem include_blank "Windows 7 32bit" "Windows 7 64bit" "Windows 8 32bit" "Windows 8 64bit" "Windows 8.1 32bit" "Windows 8.1 64bit" "Windows 10 32bit" "Windows 10 64bit"] </div>
</div>

<div class="row">
<div class="col-md-6 col-sm-5 no-padding"> Computer Control Version<sup>(1)</sup></div>
<div class="col-md-6 col-sm-7"> 
<div class="col-xs-6 no-padding-left"> [text version-1 placeholder "7.00"]</div>
<div class="col-xs-6 no-padding-right">[text version-2 placeholder "22"]</div>
</div>
</div>

<div class="row">
<div class="col-md-6 col-sm-5 no-padding">Data Processing Version<sup>(1)</sup></div>
<div class="col-md-6 col-sm-7"> 
<div class="col-xs-6 no-padding-left">[text data-version-1 placeholder "3.00"]</div>
<div class="col-xs-6 no-padding-right">[text data-version-2 placeholder "00"]</div>
<p class="comt"><span class="note"><sup>(1)</sup>To obtain the precise version, start WinSPM and select Help->About from the top menu.</span></p>
</div>
</div>

<div class="row">
<div class="col-md-6 col-sm-5 no-padding"> Name <span class="vlabel">(required)</span></div>
<div class="col-md-6 col-sm-7"> [text* your-name] </div>
</div>

<div class="row">
<div class="col-md-6 col-sm-5 no-padding"> Company <span class="vlabel">(required)</span></div>
<div class="col-md-6 col-sm-7"> [text* your-company] </div>
</div>

<div class="row">
<div class="col-md-6 col-sm-5 no-padding">  Email <span class="vlabel">(required)</span> </div>
<div class="col-md-6 col-sm-7">  [email* your-email] </div>
</div>

<div class="row">
<div class="col-md-6 col-sm-5 no-padding">  Subject <span class="vlabel">(required)</span></div>
<div class="col-md-6 col-sm-7"> [text* your-subject] </div>
</div>

<div class="row" id="your-description">
<div class="col-md-6 col-sm-5 no-padding"> Description <span class="vlabel"></span> </div>
<div class="col-md-6 col-sm-7"> [textarea your-description cols:40 rows:5] </div>
</div>

<div class="row" id="expected-result">
<div class="col-md-6 col-sm-5 no-padding"> Expected Result<span class="vlabel"></span> </div>
<div class="col-md-6 col-sm-7"> [textarea your-expected-result cols:40 rows:5] </div>
</div>

<div class="row" id="actual-result">
<div class="col-md-6 col-sm-5 no-padding"> Actual Result<span class="vlabel"></span> </div>
<div class="col-md-6 col-sm-7"> [textarea your-actual-result cols:40 rows:5] </div>
</div>

<div class="row">
<div class="col-md-6 col-sm-5 no-padding"> Upload files (e.g. images, spectra, screenshots, logs) </div>
<div class="col-md-6 col-sm-7"> [multifile multifile-53 limit:24MB id:filesToUpload1 filetypes:3dp|7z|aft|asc|avi|bft|bin|bmp|c|cfg|clb|cnt|col|cpp|csv|dat|dib|dmp|doc|docx|dump|dpt|epd|gif|h|hdr|htm|html|ico|ini|jfif|jpe|jpeg|jpg|lnt|log|m4v|mac|map|mkv|mp4|mpd|mpg|mpeg|msk|pdf|pls|png|ppt|pps|pptx|ps|psd|rar|reg|rpd|rpt|rtf|spc|spd|stg|tar.gz|tif|tiff|tmp|trc|txt|vec|wmv|xls|xlsm|xlsx|z|zip]
<ul id="fileList"><li>No Files Selected</li></ul>
<div class="multifile_container"></div>
</div>
</div>
<div class="row">
<div class="col-md-6 col-sm-5 no-padding"> &nbsp; </div>
<div class="col-md-6 col-sm-7">
[recaptcha]
[submit "Submit"]</div>
</div>

<script language="javascript" type="text/javascript">
// Hide the Text field by default
jQuery(document).ready(function(){
document.getElementById('expected-result').style.display = 'none';
document.getElementById('actual-result').style.display = 'none';
});
document.getElementById('request_type').addEventListener('click', displayTextField);
function displayTextField() {
  // Get the value of the currently selected radio button. 'select-a-size' is the name of the radio buttons you specify in the form builder
  var radioText = document.querySelector('input[name="type"]:checked').value;
  if (radioText == 'Bug Report') {
    document.getElementById('actual-result').style.display = 'block';
 document.getElementById('expected-result').style.display = 'block';
 document.getElementById('your-description').style.display = 'none';
  } else {
        document.getElementById('actual-result').style.display = 'none';
 document.getElementById('expected-result').style.display = 'none';
 document.getElementById('your-description').style.display = 'block';
  }
}jQuery('#filesToUpload').change(function() {		
jQuery('#fileList').show();
			var input = document.getElementById("filesToUpload");
			var ul = document.getElementById("fileList");
			while (ul.hasChildNodes()) {
				ul.removeChild(ul.firstChild);
			}
			for (var i = 0; i < input.files.length; i++) {
				var li = document.createElement("li");
				li.innerHTML = input.files[i].name;
				ul.appendChild(li);
			}
			if(!ul.hasChildNodes()) {
				var li = document.createElement("li");
				li.innerHTML = 'No Files Selected';
				ul.appendChild(li);
			}
});
jQuery(function($)
    {
      $('#filesToUpload1').multifile();
    });
</script>

I advise you to stop using the “Multifile Upload Field for Contact Form 7” plugin and create a new contact form from the default templates.

See Getting started with Contact Form 7

If you want to continue using the plugin at any cost, ask for help in the plugin’s support forum.