Want to copy attachments from one list to a other

Well, you do not give too much information about why and how you want to achieve your goal. Do you need to copy list item, attachments or both? How to choose a destination? SharePoint 2013 or 2010 (or destination is different)? Is workflow definitely a good choice? Maybe just ribbon action is enough.

Anyway. 'Attachments' field is actually a boolean field - true if list item has attachments, false - if not. You will not have much benefit of it. I would use some JavaScript action:

var url = _spPageContextInfo.webAbsoluteUrl +
  "/_api/web/lists/getbyid('"+ _spPageContextInfo.pageListId +
  "')/items(" +
  SP.ListOperation.Selection.getSelectedItems()[0].id +")/AttachmentFiles";
$.ajax({
  url: url,
  method: "GET",
  headers: { "Accept": "application/json; odata=verbose" },
  success: function (data) {
    var html = [];
    if (data.d.results.length > 0) {  
      $.each(data.d.results, function () {  
        html.push("<div><a href='" + this.ServerRelativeUrl + "'>" + this.FileName + "</a></div>");
      });  
    }
    updateListItem(html.join(''));
  },
  error: function () { alert("failure"); }
});

function updateListItem(html) {
$.ajax({
  url: _spPageContextInfo.webAbsoluteUrl + "/_api/web/lists/GetByTitle('DestinationListTitle')/items([provide target list item ID])",
  type: "POST",
  data: JSON.stringify({  __metadata: { type: "SP.Data.DestinationListNameListItem" }, Link: html }),  // 'Link' is field name to be updated
  headers:  {
    "Accept": "application/json;odata=verbose",
    "Content-Type": "application/json;odata=verbose",
    "X-RequestDigest": $("#__REQUESTDIGEST").val(),
    "IF-MATCH": "*",
    "X-HTTP-Method": "MERGE"
  },
  success: function()  { alert('Done') },  
  error: function() { alert('Error'); }
});  
}

Select allOpen in new window