IMO, using ".yml" - that is, restricting the suffix to three characters
- is a relic of the "bad old days" and should be avoided. FWIW, I feel
the same way about ".htm" :-)
The spec itself is silent on this issue, and for a good reason - the
question of "valid" suffixes is pretty much irrelevant in the first
place. Files suffix should reflect the *schema* rather than the
XML has the same issue - every time you use a '.html'/'.xhtml' suffix,
you are using the schema rather than the fact the file is using XML
syntax. Sure there are plenty of '.xml' files out there, but it is
hardly "the" suffix for XML files.
YAML files are expected to follow the same pattern - there would be
plenty of ".yaml" files (and, alas, ".yml" files), but there would also
be plenty of other files suffixes used, depending on the application.