Support single MultipartFile, not only List<MultipartFile>

[brun0xon]

Cannot pass single MultipartFile as Part() param. Getting generator error Exception: toJson() method have to add to MultipartFile. I can't use File type, because generator uses Platform.pathSeparator which throws in web, and can't use List<int>, because it may be a different file extension type.

[stale[bot]]

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[StarProxima]

@trevorwang Is there no progress on this problem?

Dio supports single file sending - https://pub.dev/packages/dio#sending-formdata