Include text-based documents (doc, txt, pdf, etc) in the search index. So when a search query appears inside an attached document, it shows up in the search results, this could be made optional via a checkbox.

Ideally the search results would show the context of the searched query inside each matched result and when clicked, it would show the document where the matched search query is highlighted.

Gmail already has this functionality but due to privacy concerns I'd rather use my own IMAP and mail client.