i put the following inside ~/.local/share/thumbnailers/webp.thumbnailer

[Thumbnailer Entry]
Version=1.0
Encoding=UTF-8
Type=X-Thumbnailer
Name=webp Thumbnailer
MimeType=image/webp;
Exec=/usr/bin/convert %i -thumbnail %s %o

and it doesn't work

i even put this inside ~/.local/share/mime/image/webp.xml

<?xml version="1.0" encoding="utf-8"?>
<mime-type xmlns="http://www.freedesktop.org/standards/shared-mime-info" type="image/webp">
  <!--Created automatically by update-mime-database. DO NOT EDIT!-->
  <comment>WebP file</comment>
  <icon name="image"/>
  <glob-deleteall/>
  <glob pattern="*.webp"/>
</mime-type>

and nothing, webp images inside pcmanfm still show the generic image icon

i'm on pcmanfm 1.3.2-1 with tumbler 4.16.0-1 on devuan ceres.

nevermind, i figured it out, pcmanfm uses gdk-pixbuf for images and doesn't really query thumbler for that, so it needs a webp loader for gdk-pixbuf, i'm lazy and don't use any other software that depends on gdk-pixbuf for the images so i just switched to thunar.