To enable the attachment search, simply go to the General Options -> Attachments panel and click turn on the Return attachments as results? option:
By default, the plugin will look in attachment Titles, Content and Captions fields, you can adjust these options on the same page:
Mime Types (file types)
By default .jpeg, .png, .tiff and .ico files are set to return as results, but you can add any mime (file) type of your choice. To get your desired mime type, please check the WordPress allowed mime types table table or the table in chapter below.
To allow more mime types, simply enter them to the Allowed mime types textarea option separated by commas: