Force thumbnail generation option under extra networks

[i-liek-katz]

Currently, it seems a thumbnail for an extra network file will generate when a dimension is larger than x, unsure what x is. I've had files much smaller in filesize generate a thumbnail, while some larger files be left alone. Maybe an option could be added to define what kind of parameters must be met to generate a thumbnail, or perhaps an option to always generate a thumbnail with a dimension of 512px by whatever.

[vladmandic]

rule for creating thumbnails is: if img.width > 1024 or img.height > 1024: