Use file exists for do callout

[jokoho48]

When merged, this pull request will:

1. Possible Replacement for Check callout validity #380

It is generally slower than #380, but it does not matter because it gets cached for the benefit of fewer false discards.

[rekterakathom]

Looks good. I think this one manages to find RHS callouts by adding the file extension, which is something that #380 can't do.

I added the .mp3 extension to #380 "just to be sure" since it didn't hurt performance, but here it might have a big impact on the worst-case performance (when all extensions are tested). Perhaps it's worth testing whether or not .mp3 is a valid format accepted by the engine at all, the biki says it is not.