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/text_to_speech.rs

@@ -3,6 +3,7 @@ use mxlink::{MatrixLink, MessageResponseType};
 
 use tracing::Instrument;
 
+use crate::controller::utils::mime::get_file_extension;
 use crate::{
     agent::{provider::TextToSpeechParams, AgentInstance, AgentPurpose, ControllerTrait},
     entity::MessageContext,
@@ -117,7 +118,7 @@ async fn do_generate_and_send_tts_for_message(
         }
     };
 
-    let attachment_body_text = strings::text_to_speech::alternate_body_text();
+    let attachment_body_text = format!("generated-speech.{}", get_file_extension(&text_to_speech_result.mime_type));
 
     let event_content = matrix_link
         .media()