If an ufp file is uploaded to OctoPrint the Cura thumbnails plugin replaces the file with the extracted gcode file. This new gcode file isn't automatically processed with Arcwelder.
My guess is that the method used by the Cura thumbnails plugin to save the gcode file sidesteps the point where the post-processing plugins are called, maybe because it is itself a post-processing plugin.
Is this a bug in one of the plugins or an unfortunate result of the plugin engine? Or are my expectations wrong ?