form.html

<form id="new_document_form" method="post">
  <!-- We need doc _rev in POSTed data for Couch -->
  <input type="hidden" name="_rev" value="">
  <p><input type="file" name="_attachments"></p>
  <p><button type="submit" class="add">Send</button></p>
</form>

upload.js

// Needs http://malsup.com/jquery/form/ plugin

var upload_attachment = function(form) {
  
  // Store the upload form reference
  var form = $(form);
  // Obtain new doc ID from Couch
  var doc_id = $.couch.newUUID();
  // Create new doc in Couch to hold the attachment
  var doc = { _id : doc_id,
              created_at : new Date() }
  db.saveDoc(doc,
             { success : function(response) {
                // Update the _rev input with document revision
                form.find("input[name='_rev']").val( response.rev );
                // Submit the form with file upload via Ajax
                form.ajaxSubmit({
                  url    : db.uri + doc_id,
                  type   : 'POST',
                  dataType : 'json',
                  // NOTE: ajaxSubmit has only "complete" callback, others are reserved
                  complete : function(upload_response) {
                    var result = JSON.parse( $(upload_response.responseText).text() );
                    if (result.ok) {
                      $this.message('Document successfully saved.');
                      $this.display_list( { highlight_last : true } );
                    } else {
                      $this.message('Cannot save the document. Reason: ' +
                                    '<code>'+$(upload_response.responseText).text()+'</pre>', 'error');
                    }
                  }
                });
                return false;
                },
                error : function(status, error, reason) {
                  alert('Unable to create document in the database ' + db.name +
                        ', reason: ' + reason.toString())
                }
  });
  return false;
};