[FIX] web: Safari, download of attachments with accents

The structure `filename*=UTF-8` works in Safari, from
release 6.2 (which is set as version 537 in the user_agent version).

This structure has the advantage to handle the UTF-8 encoding,
and therefore special characters, such as accents.

Therefore, from this Safari release, we use the same format
than other browser. For older releases, we use the format
`filename=`, without UTF-8, ASCII encoded, replacing
special characters by `?`.

opw-649863