Quick answer: Don't know. Longer:
It'd be nice to do if that would avoid most nameclashes, but as Michael says, <letter>db will clash. E.g. I thought of sdb (single-level store) or odb (Openldap's DB). But they clash with OpenOffice databases which is perhaps worse than with Microsoft stuff.
.mmdb (Memory Mapped DB) seems mostly unused, but perhaps 4-letter .exts still remain messier than clashes with other applications' .exts.