On OOTB SharePoint lists without a defined Nintex Form, attachments with an XML extension are presented in the UI along with all other attachments. On Nintex forms, XML attachments are omitted from display but the list of other attachments are still rendered in the UI.
Below is the HTML rendered on the Nintex form for a specific list item with three attachments (one .XML). Only two attachments are shown in the UI which is matched by the HTML below (nothing hidden in the HTML).
<table class="nf-attachmentsTable nf-default-attachment-control" id="idAttachmentsTableaec57343-30cf-4236-a23d-f900dc28ec3c" border="0" cellspacing="0" cellpadding="0">
<tr id="idAttachmentsRowaec57343-30cf-4236-a23d-f900dc28ec3cSome Attachment 1">
<td class="ms-vb"><a href="[[Some Attachment 1]]">Some Attachment 1</a><img width="10" alt="Delete" src="/_layouts/15/NintexForms/images/spacer.gif"></td>
<tr id="idAttachmentsRowaec57343-30cf-4236-a23d-f900dc28ec3cSome Attachment 2">
<td class="ms-vb"><a href="[[Some Attachment 2]]">Some Attachment 2</a><img width="10" alt="Delete" src="/_layouts/15/NintexForms/images/spacer.gif"></td>
When I run a powershell script against the list, it shows all three attachments for this list item. If I delete the Nintex form, reverting to the default SharePoint Dispform.aspx, I can see all three attachments in the UI.
Attaching an XML file is a customer requirement and saving attachements as a TXT extention will not properly render the XML when the user clicks to view: hence, a "don't do that" answer won't work. Again, it works on OOTB SharePoint lists, but not in the Nintex forms. The customer has deployed Nintex Forms 2013 ( 22.214.171.124 ). If this is a version issue, will gladly request an update.