To view thumbnails of media files in Trash open the files manager, click the options button in the title bar, turn on Show hidden files 
, open /home/(your username)/.local/share/Trash/files and wait for the thumbnails to load. Now return to Trash and you'll see them there, too. Unfortunately this doesn't make future deleted media files show their thumbnails, this trick only works for the current files 
. But it's useful to see thumbnails of tons of deleted audios, images and videos and choose which ones to delete permanently without having to open them one by one 
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