HTML Paste Clipboard Image to File Input

It's pretty straightforward. Just catch paste event on window object and put all the files you got from it to the <input> tag.

const form = document.getElementById("new_document_attachment");
const fileInput = document.getElementById("document_attachment_doc");

fileInput.addEventListener('change', () => {
  form.submit();
});

window.addEventListener('paste', e => {
  fileInput.files = e.clipboardData.files;
});

<form id="new_document_attachment" method="post">
  <div class="actions"><input type="submit" name="commit" value="Submit" /></div>
  <input type="file" id="document_attachment_doc" />
</form>

Work well

<form action="abc.php" method="post" enctype="multipart/form-data">
  <div class="actions"><input type="submit" name="commit" value="Submit" /></div>
  <input type="file" name="aaa"/>
</form>
  
 <script type="text/javascript">
//e.originalEvent.clipboardData.files
const form = document.getElementById("new_document_attachment");
const fileInput = document.getElementById("document_attachment_doc");

fileInput.addEventListener('change', () => {
  form.submit();
});

window.addEventListener('paste', e => {
  fileInput.files = e.clipboardData.files;
}); 
 </script>

PHP Output: var_dump($_FILES);

array (size=1)
  'aaa' => 
    array (size=5)
      'name' => string 'image.png' (length=9)
      'type' => string 'image/png' (length=9)
      'tmp_name' => string 'C:\wamp64\tmp\phpF6AF.tmp' (length=25)
      'error' => int 0
      'size' => int 9380