Gmail – Adding rules that filter email by their content-type in Gmail

I am currently being swamped with Russian spam so I followed several tutorials to create a custom filter but the last step is eluding me.

My current filter looks like this:

Has the words: lang:Russian OR И OR э OR ы OR я OR ю

Everything that matches the filter bypasses my inbox and gets deleted.
On top of this I would like to check for this:

charset=koi8-r

Is this doable with Gmail's filters?