In Lightroom Classic CC, if "Replace illegal file name characters with:" is set to "Similar Characters", all export presets will show a filename preview as follows: (filename).(space)(space)(file extension of original file)..(new file extension). For example, if the export is from original file Photo.tiff to jpg, the preview will show "Photo. tiff..jpg". Exports will be named correctly (without all the extraneous characters), but the extra characters are worrying when in the Export panel.