$ cat "/Users/YOURUSER/Library/Application Support/Sublime Text 3/Packages/User/YAML.sublime-settings"
{
"extensions":
[
"yml.j2"
]
}
^ That will make Sublime Text 3 default to set syntax as YAML for every .yml.j2 file. The same can be done with other extensions as well as other language/syntax.
Have fun hacking.