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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