Hi.
At the moment default character set for all opened files is charset from LANG environment variable.
But it is common, at least for Russian users, to have one system LANG (for example ru_RU.koi8r) and work with documents in another LANG/charset, for example ru_RU.cp1251.
It would be convenient to set up default charset for open files in "Preferences / Files" dialog. The same settign for New files is already there.