About [[MediaWiki:Apihelp-upload-param-leavemessage/uk]]
Fragment of a discussion from Translating talk:MediaWiki/LiquidThreads
The English message here should be "If <var>$1asyncdownload</var> is used, leave a message on the user talk page when finished.", referring to the parameter documented by Apihelp-upload-param-asyncdownload ("Make fetching a URL asynchronous.").
The feature involved is making an "upload" by having MediaWiki download the image from a URL.