We could have tweaked further the code to guess what is a meta file
procedure or not (what we have done until now) but I decided to be more
explicit and added gimp_file_procedure_[gs]et_meta() functions to
libgimp to tag a specific import/export procedure to implement a meta
format.
gimp_file_procedure_set_extensions() would still be used to list well
known formats using this meta format, for instance "hgt.zip" or
"xcf.xz,xcfxz" but the meta extension (simply "zip" or "xz"
respectively) would be used as such. No more "guessing" using the list
of extensions and assuming that at least one of them would have a dot
within.