What are we stripping? What are we keeping? Could we safely strip more than what we currently are?
As @Bawolff pointed out, we strip most metadata from PNG thumbnails but not "thumbnail metadata". Which manifests itself as these fields when inspecting a thumbnail with exiftool:
Thumb Document Pages : 1 Thumb Imageheight : 4608 Thumb Image Width : 3072 Thumb Mimetype : image/png Thumb M Time : 1455745807
@Bawolff which IM arguments do you know to be capable of removing these fields?