OK. I have learned the purpose of that @@PLUGINFILE so I'm one step closer to a solution
Though, still don't have a clue how to use file_rewrite_pluginfile_urls function so I have asked in General developer forum.
Database
SRC attribute in mdl_data_content.content
Обсуждение было заблокировано, поэтому на него нельзя больше отвечать.