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>

docs/configuration/handlers.md

@@ -8,7 +8,7 @@ You can also use **different models within the same room** (e.g. [💬 text-gene
 
 The bot supports the following use-purposes:
 
-- [💬 text-generation](../features.md#-text-generation): communicating with you via text (though certain models may "see" images as well)
+- [💬 text-generation](../features.md#-text-generation): communicating with you via text (though certain models may also process images and files)
 - [🦻 speech-to-text](../features.md#-speech-to-text): turning your voice messages into text
 - [🗣️ text-to-speech](../features.md#️-text-to-speech): turning bot or users text messages into voice messages
 - [🖌️ image-generation](../features.md#image-generation): generating images based on instructions