[Enhancement] Randomize name, as an option

[gaussandhisgun]

Some XMPP servers (say, aria-net in my personal experience) do not let you upload files with cyrillic letters in their names. Mobile XMPP clients, say, Monocles, are not affected by this because they randomize the filename before sending it, but this plugin does not do so - which causes a bit of extra inconvenience whenever I need to upload my screenshots into a chat.