This patch has been merged upstream. Drop next update.
Commit: c8b1ea28364ef185ef61d2ca20230e92bb475b89

Index: src/XDGData/FreeCAD.thumbnailer.in
--- src/XDGData/FreeCAD.thumbnailer.in.orig
+++ src/XDGData/FreeCAD.thumbnailer.in
@@ -0,0 +1,4 @@
+[Thumbnailer Entry]
+TryExec=@CMAKE_INSTALL_BINDIR@/freecad-thumbnailer
+Exec=@CMAKE_INSTALL_BINDIR@/freecad-thumbnailer -s %s %i %o
+MimeType=application/x-extension-fcstd;
