[12.x] Fix Number::fileSize() handling of negative byte values

[Amirhf1]

Number::fileSize() does not scale negative byte values to the
appropriate unit, while Number::forHumans() in the same class does.

The loop condition uses ($bytes / 1024) > 0.9, which is never true
for a negative number, so the loop never runs and the value is always
returned in bytes:

Number::fileSize(-2048);    // "-2,048 B"  expected "-2 KB"
Number::fileSize(-1536000); // "-1,536,000 B"  expected "-1 MB"