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/agent/provider/anthropic/utils.rs

@@ -28,6 +28,13 @@ pub(super) fn create_anthropic_message_request(llm_messages: Vec<LLMMessage>) ->
                     },
                 }]
             }
+            LLMMessageContent::File(file_details) => {
+                tracing::warn!(
+                    "The Anthropic provider's library does not support file/document content. This file message ({}) will be skipped.",
+                    file_details.filename(),
+                );
+                continue;
+            }
         };
 
         let message = Message { role, content };

src/agent/provider/openai/utils.rs

@@ -1,6 +1,6 @@
 use async_openai::types::responses::{
-    EasyInputContent, EasyInputMessage, ImageDetail, InputContent, InputImageContent, InputItem,
-    InputParam, MessageType, Role,
+    EasyInputContent, EasyInputMessage, ImageDetail, InputContent, InputFileArgs,
+    InputImageContent, InputItem, InputParam, MessageType, Role,
 };
 
 use crate::conversation::llm::{
@@ -35,6 +35,21 @@ pub fn convert_llm_messages_to_openai_response_input(
                     file_id: None,
                 })])
             }
+            LLMMessageContent::File(file_details) => {
+                let file_data = format!(
+                    "data:{};base64,{}",
+                    file_details.mime,
+                    base64_encode(&file_details.data)
+                );
+
+                let file_content = InputFileArgs::default()
+                    .file_data(file_data)
+                    .filename(file_details.filename())
+                    .build()
+                    .expect("Failed to build InputFileContent");
+
+                EasyInputContent::ContentList(vec![InputContent::InputFile(file_content)])
+            }
         };
 
         items.push(InputItem::EasyMessage(EasyInputMessage {

src/agent/provider/openai_compat/utils.rs

@@ -40,6 +40,12 @@ fn convert_llm_message_to_openai_message(llm_message: LLMMessage) -> Option<Mess
             );
             None
         }
+        LLMMessageContent::File(_file_details) => {
+            tracing::warn!(
+                "The OpenAI-compat provider's library does not support file content. This file message will be skipped."
+            );
+            None
+        }
     }
 }