use 50KiB instead of 50kB as file size limit

messages like "Adding big file (49KiB > 50kB)" are a tad confusing.

Task-number: QTQAINFRA-485
Change-Id: If3ce78b47be8ba2e64075553abab7236044fb57a
Reviewed-by: Sergio Ahumada <sergio.ahumada@digia.com>