When speech-to-text/flow-type = `only_transcribe`, the bot will now send
text messages by default, not notices.
While notice messages may be less desirable with other bots in the room,
it's probably a better default for most people who enable "transcribe-only" mode.
This is an improvement related to https://github.com/etkecc/baibot/issues/14
Various clients (including newer versions of Element Web), do not like
it when the `body` field of the attachment is not a file name.
For images, a preview may not be shown and downloading the attachment
may suggest that the whole long text is used as a filename (which is odd).
There is value (improved accessibility, etc.)
in adding better descriptions (especially to generated images),
but given that it's currently problematic, I'm getting rid of it.
It's better and safer if we stick to using filenames.
