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/llm/entity.rs

@@ -1,5 +1,7 @@
 use chrono::{DateTime, Utc};
-use mxlink::matrix_sdk::ruma::events::room::message::ImageMessageEventContent;
+use mxlink::matrix_sdk::ruma::events::room::message::{
+    FileMessageEventContent, ImageMessageEventContent,
+};
 use mxlink::mime::Mime;
 
 use crate::agent::provider::ImageSource;
@@ -48,10 +50,35 @@ impl From<ImageDetails> for ImageSource {
     }
 }
 
+#[derive(Debug, Clone)]
+pub struct FileDetails {
+    pub event_content: FileMessageEventContent,
+    pub mime: Mime,
+    pub data: Vec<u8>,
+}
+
+impl FileDetails {
+    pub fn new(event_content: FileMessageEventContent, mime: Mime, data: Vec<u8>) -> Self {
+        Self {
+            event_content,
+            mime,
+            data,
+        }
+    }
+
+    pub fn filename(&self) -> String {
+        self.event_content
+            .filename
+            .clone()
+            .unwrap_or(self.event_content.body.clone())
+    }
+}
+
 #[derive(Debug, Clone)]
 pub enum MessageContent {
     Text(String),
     Image(ImageDetails),
+    File(FileDetails),
 }
 
 impl PartialEq for MessageContent {
@@ -62,6 +89,7 @@ impl PartialEq for MessageContent {
                 // We can probably do better than this by inspecting `.event_conten1t.source`, but for now this is good enough.
                 a.filename() == b.filename()
             }
+            (MessageContent::File(a), MessageContent::File(b)) => a.filename() == b.filename(),
             _ => false,
         }
     }