Does anyone have any experience sharpening the chain on a Makita 7104L?

I've tried a chain saw file, but I didn't seem to accomplish much. I have an attachment for my Dremel, but I want to be a little cautious with that.

Do any of you sharpen your own 7104L chain? If so, any tips?

The other option is to send it out for sharpening. I tried one chain saw shop, but they weren't interesting in trying to sharpen this.

Can people who sharpen chain saw chains typically sharpen a mortiser chain?

If I need to send it out, where should I send it?

Is there any place where I can buy a second chain for less than $325? (To use while the first is being sharpened.)

Thanks!

Jon