WecomMessageConverter.yiri2target() always emits {'media_id': ...} for
image/voice/file parts (never 'media'), matching the correct usage
already in reply_message(). send_message(), the entry point plugins
use via PluginToRuntimeAction.SEND_MESSAGE, instead read
content['media'], which is never set, so any image/voice/file part
raises KeyError and aborts the send.
Refs #1687
Signed-off-by: Amir Fathi <amirfathi.me@gmail.com>