UploadsMiddleware discards non-text content blocks in multi-modal messages

Description

DeerFlow's `UploadsMiddleware` at `uploads_middleware.py:183-188` only extracts content blocks where `type=='text'`. Image blocks and other non-text content are silently dropped before the message is reconstructed and passed downstream.

Reproduction

# uploads_middleware.py:183-188:
# Only type=='text' blocks extracted
# Image blocks dropped before reconstructing message