When a file's name contains non-ASCII characters, the file is incorrectly reported both as not used and as missing by the plugin. I also tested this on qa.moodle.net, and the problem persists.
To reproduce :

• create a new label in a course
• using the Atto editor, add new image
• upload a PNG file named 'éxample.png'
• check that the image displays correctly in the editor, and that saving the label yields the expected result
• edit the newly created label again
• check that the image still displays correctly in the Atto editor
• in Atto, click the "Manage files" button
• check that the image file is present in the file manager
• note that the file is wrongly reported as not being used
• note the a file with bizarre encoding artifacts is wrongly reported as being linked but not present