simple channel extract from a .mpo

[takeru]

By "export" function, we are allowed to extract one of the channels from a .mpo file.

However, we are asked to specify the image quality, and it looks a re-encoding is taking place.
As .mpo is a simple JFIF container to hold multiple JPEG images, I would like to have an option to simply extract contained image without any loss of the quality.

Kiitos,