In the case of captioned images, the `body` of the event is the caption,
and is ill suited to be a filename. Often the caption can be
prohibitively long, causing the media to fail to load in some cases.
Fixes: https://github.com/element-hq/element-x-android/issues/2882
Signed-off-by: Joe Groocock <me@frebib.net>
5820e06e0b