JPEG option for thumbs

Hi

Since I noticed that my thumbs directory is too huge,
I added an option that using jpeg as thumb files format.

It reduces file size around 4-6 times and 4x faster!.
Even it lacks alpha and the tEXT used the Thumbs(app purging thumbs)
it is worthy to be an option.


My fork is here:
https://github.com/jun7/rox-filer

Thanks.


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