Files and folders downloader add-on with resume support.
Ok, I solved this. It seems that the HTTP header "Content-Type: text/html; charset=utf-8" (the charset part) is mandatory to be issued by the server when outputting the list of files to be downloaded. Seemed to me a bit unnecessary, as long as you are instructing the Java applet to expect an utf-8 encoded output.
Also, it turns out that the filename used when saving the files is the one from the HTML code, from the list of files and the actual filename provided by the server when the file is being downloaded is completely ignored. Just so people know.
P.S. The "meta" HTML tag doesn't seem to make any difference.