In this case
You're absolutely right, even though only a single question uses the tag as referring to the file extension.
The leading dot makes it clear that it refers to a file extension, so there's absolutely no way it could stand for something else (as long as there's no well-known Unix dot file, like .bashrc, by that name). It's likely just an accident due to suggestions in tag name autocompletion.
It depends on what is considered on-topic on the site, and what's the more common interpretation on this site. We recently had this with kvm, which usually stood for keyboard/video/mouse switches, but more recently also stands for kernel-based virtual machine. Ubuntu.SE or Unix.SE will most likely judge differently.
It's not always what was earlier though: I believe virtual memory is (at least in consumer computing) older than virtual machines, but uses of vm almost always refer to the latter.
I also remember a discussion about sas (Serial Attached SCSI and a statistics program). Questions about either are usually better suited for other Stack sites though.
It's possible that a (rarely used) tag has its meaning changed when a new technology or product comes along, especially if there's no tag wiki. But the community of Meta.SU must decide about ambiguous tags case by case.