You can link to any url you want, including file shares, as long as it is presented as a proper url, not a "share".
I suspect what you've seen about it not being possible is the cases where people are trying to link to a file on the Confluence server and expecting c:\data\myfile to work - not understanding that their browser sees that as a path to files on the client machine.
You need the URI of the share. For example, let's say you have the following situation:
The URI you need to link to is \\mycomputer\shared\file.txt
If your users are not connected to your corporate network then you'll need to share a file from a public-facing service like Dropbox, OneDrive or Google Drive. Those services will provide a URL that anyone can click on and access.
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