Are you sure that you need to do this? At least on EZ 3.8.3 the file is stored in the var directory under a name like you mentioned, but it I use the default template for displaying a link to this file, I get a link to /content/download/<object_id>/<attribute_id>/file/original_filename.mp3
So if you have a look at the template design/base/override/templates/datatype/ezbinaryfile.tpl, you can see how they create a link with the original filename.
Just have a look at the template for ezbinary, there you can see how to create a link for downloading a binary file. The result should look roughly like this (not tested at all):
<a title="Télécharger {attribute_view_gui attribute=$p.data_map.titre} - (clic droit, puis Enregistrer la cible sous)"
href={concat("content/download/",$p.data_map.son.contentobject_id,"/",$p.data_map.son.id,"/file/",$p.data_map.son.content.original_filename)|ezurl}
style="margin-right:3px;width:30px;height:30px;float:right;display:block;">
{$p.data_map.son.content.original_filename|wash(xhtml)}</a>
