Send a file message to an active conversation via a url

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BSUID Support: The conversation target parameter now supports additional identifier formats:

Format	Example	Description
Conversation ID	685bd235e6119686e693a093	Direct conversation lookup (existing)
Phone Number	14155552671	Most recent open conversation for phone (existing)
Contact ID	507f1f77bcf86cd799439011	Most recent open conversation for contact (new)
BSUID	US.123124141512	Most recent open conversation for BSUID (new)
Channel:Phone	MyChannel:14155552671	Channel-scoped conversation (existing)
Channel:ContactId	MyChannel:507f...	Channel-scoped by contact ID (new)
Channel:BSUID	MyChannel:US.123124141512	Channel-scoped by BSUID (new)

Note: Returns 409 Conflict if multiple open conversations exist for the contact without a channel specified.