Populate image/audio attachment body with a filename, not with text

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.

src/controller/utils/mod.rs

@@ -7,6 +7,7 @@ use crate::{
 
 pub mod agent;
 pub mod text_to_speech;
+pub(super) mod mime;
 
 pub async fn get_text_body_or_complain<'a>(
     bot: &Bot,