Hi Marc,
Well, the documents attachment form control can be found in
~\CMSModules\Content\Controls\Attachments\DocumentAttachments\ location and is using UniGrid internally, so here you can find information on how to customize the UniGrid control on your own:
http://devnet.kentico.com/docs/controls/unigrid_getting_started.htmBasically, it would be about modification of
~\CMSModules\Content\Controls\Attachments\DocumentAttachments\DocumentAttachmentsList.xml file in your case.
It is possible to access any form control from CMSForm control, that is not a problem with Kentico
CMS API, but the only problem in case of DocumentAttachments control is that you would need to customize it and expose some additional properties to this form control (like for example UniGrid) in order to elegantly work with it.
Best regards,
Miro