When you create a snippet without specifying any scope it will be active for every file type:
<snippet>
<content><![CDATA[
Hello, ${1:this} is a Sublime Text ${2:snippet} for all scopes.
]]></content>
<tabTrigger>hello_all_scopes</tabTrigger>
</snippet>
To make the snippet visible as a popup on auto completion add the scope to the user preferences:
{
"auto_complete_selector": "text"
}
Note that the key auto_complete_selector has a default setting that is overridden by the user key. You'll most likely want to add the default settings in your custom configuration. Scope selection can be further refined or other scopes added to the list, according to your requirements.
<scope>tag in it but it still doesn't work in plain text files?auto_complete_selectorwith no scope. I recreated the configuration on a pristine install and it is working for me.