cubicweb-file #2885868 thumbnail generation crashes [validation pending]

...when the format is unknown (or wrongly computed)
I propose to always provide a png.

prioritynormal
typebug
done in1.14.0
load0.100
load left0.000
closed by#d25b316fe1f1 [entities,hooks] separate thumbnailing from resizing
patch[entities,hooks] separate thumbnailing from resizing [applied][entities] always save the thumbnail as a png (closes #2885868) [applied]