Parse "next part" and attachments

[marceltaeumel]

At the end of the body text, there can be additional parts to be downloaded from pipermail such as HTML content, images, or changesets:

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URL: <http://lists.squeakfoundation.org/pipermail/squeak-dev/attachments/20201012/c2999c36/attachment.html>
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Name: WebClient-preauth-default.1.cs
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