Hai messo (almeno) un fine boundary di troppo.

La sintassi corretta è (ad esempio):

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Content-Type: image/jpeg;
name="1.jpg"
Content-Transfer-Encoding: base64
Content-Disposition: attachment;
filename="1.jpg"

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Hh4eHh4eHh4eHh4eHh4eHh4eHh4eHh4eHh4eHh4eHh4eHh4eHh 4eHh4eHh7/wgARCAIDAjcDASIA

------=_NextPart_000_000B_01C5E9DE.E522C5C0
Content-Type: image/jpeg;
name="2.jpg"
Content-Transfer-Encoding: base64
Content-Disposition: attachment;
filename="2.jpg"

/9j/4AAQSkZJRgABAQEASABIAAD/2wBDAAUDBAQEAwUEBAQFBQUGBwwIBwcHBw8LCwkMEQ8SEhEP
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OqTqk6pOqTqk6pOqTqk6pOqTqk6pOqTqk6pOqRCsulY5FCjMaO cd6G2prBLrLJ/7U7Jlf//Z

------=_NextPart_000_000B_01C5E9DE.E522C5C0--