[storage] missing qrefresh in previous patch applied: fix comment, error message, and use a storage specified encoding, not cubicweb's encoding

[fix] fix path unicity process in BytesFileSystemStorage.new_fs_path The previous implementation was bugged (prefixing the whole path with '_' instead of the base name). A new version (using number) replace it. * * * Improve BytesFileSystemStorage.new_fs_path to use available metadata This version try to get some hint about how to name the file using metadata. Using the real file name and extension when available. Keeping the extension might be usefull for exemple in the case of processing that use filename extension to detect content-type.

[source] small refactoring to avoid being doomed by a wrong assertion message on buggy storage raising attribute error