Dear Maxim,
it works like a charm now.

FYI I put in nginx.conf:
[code]
...
if ($request_uri ~ "(*UTF8)^(.*)[\\?|&]filename=([% ,a-zA-Z0-9\x{386}-\x{3ff}_\-\.]+)(&.*)?$") {
...
[/code]

Kindest regards,
Tilemahos Manolatos

PS. @Weibin Yao: I would like to avoid "fixed" character lists, I wanted to use ranges of characters, so the above solution seems, in my opinion, better for matching all Greek characters