fix: handling attachment when it is the only part of the message

[madbob]

This is a pretty optimistic solution to an issue reported multiple times.

The assumption is that if the base structure of the message is not a multipart, nor an intelligible body, then the message is just composed by an attached file to be displayed.

Fixes #4423
Fixes #10752
Fixes #11673

[kesselb]

Thanks for your PR

[ChristophWurst]

Makes sense

The only remark is that hasAttachments is probably wrong so such email wouldn't have the paperclip icon. That needs a fix in ImapMessageFetcher::hasAttachments.

[ChristophWurst]

/backport to stable5.10

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[madbob]

I don't think over-complicating hasAttachments() for an edge-case may be the best, given that it is also a recursive function and track there the specific case of the root message's part composed only by one file may become an headache.

Here I've just enforced the value of $this->hasAnyAttachment when there are no other chances.

Fun fact: I've accidentally moved hasAttachments() after the decryption of encrypted messages, only to realize that calling it before (in the original place) may produce broken results as the encrypted message has not yet been splitted in his MIME parts.

(Tests randomy failed on the latest push due external motivations)