Till now we have Search Mail search box available in Gmail which allows to search for email messages, you can turn Search Mail in to “Search Mail and Docs” by enabling Apps Search Gmail labs feature which allows to find email messages and docs quickly.
There are no more worries to find whether the attachment in email message or in Google Docs with App Search labs feature you can able to quickly search for email messages and docs in Gmail simultaneously at a time and search results will include matching documents and sites in addition to email messages.
To enable App Search Gmail labs feature visit Gmail Labs under settings and select and enable Apps Search feature .After enabling Apps Search you can observe Search Mail and Docs search option in Gmail instead of Search Mail.(via)