Is there a feature/addon that I can use to allow users to search directly for attachments? I’m specifically looking to find images at the moment, I envision something like images.google.com that returns a mosaic of images - only internal to the Discourse of course.
You can’t return results as an image gallery, but you can search for posts containing uploads with specific extensions.
Type filetypes:pdf in the search field 
You can search for multiple file extensions with filetypes:pdf,png,mp3, for instance.
It’s not available directly from the advanced search form and is found here:
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