Wow this is pretty awesome! A few weeks ago we discussed if there was any easy way to verify your authorship of a photo (or other non-text content) and now we see some compelling evidence that Google is serious about letting us claim non-HTML content:

• PowerPoint files -> include "by Your Name" on each slide
• Excel -> include "by Your Name" tab in the workbook, and Google uses the tab name as the title of the page.
• Word documents -> include "by Your Name" (Janet Driscoll Miller says that simple the "About the Author" part may help)
• *Some* Image files -> byline was included in the SVG file in its text.
• Google Docs -> the same "by Your Name" pattern works here

As far as you can see, Google is even more heavily relying on "by Your Name" pattern nowadays (even though they fail miserably too often with it)