I don't believe that coding syntax would qualify as an end-user usability issue. As a file name, I use underscores all the time, but only at the file system level. On a webpage, if the file name in question is hyperlinked, the hyperlink underline (either persistent or on hover) would obscure the underscore, like so <file_name>.
I would agree, that if the file name is visible to the user, avoid underscores, especially if it's hyperlinked.
Sent from my mobile device. Please excuse typos and misspellings.
On Aug 8, 2012, at 2:20 PM, Kate Pendley <[address removed]> wrote: