Support binary files from base64 encoding

[tony-aq]

This change is 

[atis]

This would be very needed, right now i'm using workaround:

var event2 = event;
if (event.isBase64Encoded) {
    event2.body = Buffer.from(event.body, 'base64').toString('binary');
}
var files = multipart.parse(event2, false);

[arackaf]

@atis - solid - thanks a ton. Also, it's not entirely clear what that spotText field does, but it seems true is what you now want to if you want content to be a buffer, which is the only way this works for me.

[MMIharish]

@atis - Thanks. For me also, this worked with multipart.parse(event2, true);