How to whitelist a specific extension

How to whitelist a specific extension? (Such as https://example.com/book.pdf)

1. Open the extension settings page
2. Go to the whitelist settings, and create a new line
3. Copy this code /([a-zA-Z0-9\s_\\.\-\(\):])+(.pdf)$/ and change .pdf to the extension you want to whitelist. If you want multiple extensions, add a vertical bar (|) and add the extension after it. Example: /([a-zA-Z0-9\s_\\.\-\(\):])+(.doc|.docx|.pdf)$/
4. Paste it into the whitelisting box and any websites that have the extension you chose will be whitelisted.