Is there any reason to keep the globals options "thumbnails_png" in the "GFX" settings?
This option is used in the method ("determineTargetFileExtension") in the file "sysext/core/Classes/Resource/Processing/AbstractGraphicalTask.php".
It's a problem for example if you have "thumbnails_png" set to true and animated gif. The gif will be converted to png and the animation will gone.
For me the conversion must be set only for non image file (like pdf). So a gif, jp(e)g, png etc. must not been converted.