Open a page for editing. Place the cursor in the text. Click Insert > Media.
Try typing various beginnings of sensible search term snippets in the search field without hitting Enter, for instance "ark" (which should match names containing the word "Arkansas", among other things).
Sometimes the "throbber" (progress indicator) first appears during typing, and then relevant matches are shown. But sometimes the throbber is not shown, and no results either, but only the text "No results found.", although there are certainly lots of media with names matching the text snippet currently in the text box.
You will have to try this out yourself a few times, with different text snippets, because the behaviour is erratic and random. I have a suspicion the severity of the problem is related to the quality of the Internet connection used, but I haven't been able to verify this.
(Safari 11.0.3)