cubicweb-file #3467585 [migration] 1.14 yields a MemoryError crash [validation pending]

Either because this leaks or there are big files.

prioritynormal
typebug
done in1.17.0
load0.100
load left0.000
closed by#06d34ad513f1 [migration/1.14.0] try to be more memory efficient (closes #3467585)
patch[migration/1.14.0] try to be more memory efficient (closes #3467585) [applied]