Fix wrong default for maximum size of generated files

In commit e8c4ca6c a maximum size for generated files was introduced
(e.g. used in the cropping module). The intendet default was 50MB, but a
byte representation of 51200 was set. This commit changes this to
52428800, which is the correct number of bytes in 50MB.

Thanks Felix von Zadow (@fzadow) for the pointer!