dictionaries

"dictionaries": {
  "en-US": "dictionaries/en-US.dic"
}

The dictionaries key specifies the locale_code for which your extension supplies a dictionary. Although the dictionary consists of two files, one with a .dic and one with an .aff file extension, only the one with the .dic extension is referenced in the manifest.json.

If you use the dictionaries key, you must also set an ID for your extension using the browser_specific_settings manifest.json key.