For sharing multiple files, it is often convenient to pack them into an archive, and the most widely supported format to do so is probably ZIP. Under *nix, you can archive a directory with Info-ZIP:
% zip -r something.zip something/
(When you have several files, it is recommended to archive them in a directory, to avoid cluttering the directory where people will extract them.)
Unsupported ZIP archive
Unfortunately, while we would expect ZIP files to be widely supported, I found out that this is not always the case, and I had many recipients failing to open them under operating systems such as iOS.
Avoid extra fields
That issue seems to be linked to the usage of extra file attributes, that are enabled by default, in order to store Unix file metadata. The field designed to store such extra attributes was designed from the beginning so each implementation can take into account attributes it supports and ignore any other ones, but some buggy ZIP implementation appear not to function at all with them.
Therefore, unless you actually need to preserve Unix file metadata,
you should avoid using extra fields. With Info-ZIP, you would have to
add the option -X:
% zip -rX something.zip something/