youtube: add support for passing passing http url
gains: add '-f' option to force recalculation
decompr: cosmetic
decompr: fix tar.xz archives support
decompr: fix .tar archives support
decompr: fix .jar archives support
decompr: improve archive type detection

Fallback to guessing by extension if using 'file' does not work; since
the mime type returned can be invalid/different depending on the
version...