#2676 [Feature]: attach PDF, Word, Excel and PowerPoint files in the chat page

说明

### What do you want to do?

Attach a PDF, Word, Excel or PowerPoint file in Strata's chat page and ask about it, the way a text file can be attached today.

Today I convert the file to text myself (or run a separate MarkItDown MCP server) and attach the result. Doing it in the page would make it one step.

A way to do it, as an opt-in that changes nothing unless installed: the server turns the file into Markdown with MarkItDown (an optional dependency, not installed by setup) and the page attaches the text. I have a branch for this and will open a pull request linked here, in case it is wanted. If you would rather keep attachments on the client side, say so and I will close it.