It seems to me that something is trying to convert your attachment to readable text and replaces extended ascii characters to a replacement character. I don't know much about RT, however a quick look at RT::Client::REST::Attachment shows some attributes that could be of interest for you which are: content_type and content_encoding. Maybe you can try and see what these attributes return.

In reply to Re: RT::Client turns occasional binary characters in to wide characters by Veltro
in thread RT::Client turns occasional binary characters in to wide characters by wardmw

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