Create a placeholder for a binary blob

Return value
	type	description
	blob	The reference to the created blob
Parameters
name	type	description
session_id	session	valid session reference
mime_type	string	The mime-type of the blob. Defaults to 'application/octet-stream' if the empty string is supplied
Errors
name	arguments	description

• Message has only a synchronous version
• This is a custom message