As kilinrax has stated, the Disposition line is very important for this to work right. I would also recommend adding in some error checking. It's not your usual error checking, but simple enough:

my $result = $sender->SendFile( { blah, blah } );

if (!ref $result) {
  die "Sendfile failed: $Mail::Sender::Error\n";
  
}
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The most common error you'll catch this way is "file not found" but who knows? maybe it will catch the wrong (or missing) Disposition problem as well.