Default settings are set to html/php >> css/scss/less/sass/stylus flow.
To change file types you want to get selectors from use HTML to CSS autocompletion extension configuration from command palette or VSCode user settings.

• Intellisense suggestions/completions for classes and ids from markup documents to CSS, SCSS, Less, Sass, Stylus stylesheets
• Configuration of file types, files, folders or workspaces to work with

• Enter HTML to CSS autocompletion: Extension Configuration from the command palette
• Or find html-to-css-autocompletion configuration entries in VSCode user settings

• html-to-css-autocompletion.triggerCharacters Shows completion list only on '#'/'.' character entries. Default: disable

• html-to-css-autocompletion.autocompletionFilesScope
  Defines scope for extension to work with. Options:
  multi-root: all selectors found within all root folders will be visible to defined stylesheets. This is Default autocompletion provider's scope.
  workspace: all selectors found within particular workspace folder/project will be visible to stylesheets within that workspace folder.
  linked files: selectors will be provided only for linked stylesheets.

• html-to-css-autocompletion.getSelectorsFromFileTypes
  Defines file types to be searched for classes and ids. Default: html, php

• html-to-css-autocompletion.folderNamesToBeIncluded
  Defines only specific folder names to be searched. Default: ''

• html-to-css-autocompletion.folderNamesToBeExcluded
  Defines folder names to be excluded from being searched. Default: node_modules

• html-to-css-autocompletion.includePattern
  Set custom glob pattern to get classes/ids from matched files. E.g.: **/{folderName1,folderName2,...}/*.{fileType1,fileType2,...}

• html-to-css-autocompletion.excludePattern
  Set custom glob pattern to exclude search on pattern matches. E.g.: **/{folderName1,folderName2,...}/**