Well, if it were a full InfoPath form, you could add code that would:
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Decode the file attachments and get the name of each
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Copy the file attachment nodes
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Sort the file attachment nodes
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Delete the original nodes
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Append the copied nodes
However, if you do this as the user is attaching the nodes, I'd think the user experience would be pretty poor - having things move around on you while you are filling out a form is kind of disconcerting - same with if you tried to address this in the XSL only. It is a requirement I'd probably push back on a bit, frankly.