As mentioned really only two options. 1/ message them asking for an email address then send in the normal way. 2/ put the file into a file sharing web service then message them a link that will allow them to pull down the file.
At the end of the day this web site has a messaging service, not a general purpose email system. Think of it as the equivalent of an old fashioned SMS text message and you'll not be too disappointed.
Jeff
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