If cowboy_static is initialized with `{priv_file, ...}` or `{priv_dir,
...}`, it is now able to read files from Erlang application .ez
archives.
When serving a file from an archive, the #file_info{} comes from the
archive, not the contained file, except for the size and type. The
erl_prim_loader module is used to read the latter's #file_info{} and the
actual file content (ie. sendfile(2) is not used in this case).
(cherry picked from commit 2166733628)
