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Unfortunately during development process, when units are copy/pasted to easily create more units it happens, that "upload_dir" field option contains value from copied unit and that results in mixing files from different non-related units into same folders.
Solution
In the "kUploadFormatter::PrepareOptions" method when "upload_dir" option is missing and "kUploadFormatter::$DestinationPath" property is empty set it by combining "WRITEBALE_BASE" constant and unit folder.