Add `mime_type` to the returned `ByteStream`s from `LibreOfficeFileConverter`

[sjrl]

We recently released the LibreOfficeFileConverter in #3006

While working on the integration page I realized that we don't populate the mime_type field in the ByteStream dataclass. This info isn't required but is helpful to have so we should update the converter to populate the mime_type field in the returned ByteStreams.

[StealthTensor]

Hey @sjrl, I'd love to take this on! Could you assign it to me?

[sjrl]

@StealthTensor go ahead and work on it! We don't assign issues to external users but I'll update the issue to be in progress.