Add support for file attachments (m.file) in conversations

Files sent as m.file Matrix messages are now downloaded, MIME-detected,
and forwarded to LLM providers alongside the conversation context,
similar to how m.image is already handled.

- OpenAI provider: sends files inline as base64 data URLs
- Anthropic provider: skips files with a warning (library limitation)
- OpenAI-compat provider: skips files with a warning (library limitation)

Controller routing respects the existing prefix requirement setting.
MIME detection expanded to cover PDF, text, code, and document formats.
Docs updated to reflect file support and known limitations.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

src/conversation/matrix/utils/mod.rs

@@ -234,6 +234,7 @@ pub async fn convert_matrix_native_event_to_matrix_message(
         MessageType::Text(text_content) => (text_content.body.clone(), false),
         MessageType::Notice(notice_content) => (notice_content.body.clone(), true),
         MessageType::Image(image_content) => (image_content.body.clone(), false),
+        MessageType::File(file_content) => (file_content.body.clone(), false),
         _ => return Ok(None),
     };
 
@@ -291,6 +292,49 @@ pub async fn convert_matrix_native_event_to_matrix_message(
         }));
     }
 
+    if let MessageType::File(file_content) = &room_message.msgtype {
+        let media_request = mxlink::matrix_sdk::media::MediaRequestParameters {
+            source: file_content.source.to_owned(),
+            format: mxlink::matrix_sdk::media::MediaFormat::File,
+        };
+
+        let file_name = file_content
+            .filename
+            .clone()
+            .unwrap_or(file_content.body.clone());
+
+        let mime_type = file_content
+            .info
+            .as_ref()
+            .and_then(|info| info.mimetype.clone())
+            .and_then(|mimetype| mimetype.parse::<mxlink::mime::Mime>().ok())
+            .unwrap_or_else(|| get_mime_type_from_file_name(&file_name));
+
+        tracing::debug!("Determined mime type {} for file {}", mime_type, file_name);
+
+        let span = tracing::debug_span!("get_media_content", file_name = %file_name, mime_type = %mime_type);
+
+        let media_bytes = matrix_link
+            .client()
+            .media()
+            .get_media_content(&media_request, true)
+            .instrument(span)
+            .await?;
+
+        tracing::debug!(
+            "Downloaded {} bytes for file {}",
+            media_bytes.len(),
+            file_name
+        );
+
+        return Ok(Some(MatrixMessage {
+            sender_id: matrix_native_event.sender().to_owned(),
+            content: MatrixMessageContent::File(file_content.clone(), mime_type, media_bytes),
+            mentioned_users,
+            timestamp,
+        }));
+    }
+
     Ok(Some(MatrixMessage {
         sender_id: matrix_native_event.sender().to_owned(),
         content: if is_notice {