[ruby] Use Marcel to detect content-type in form-data file upload using faraday

[ykitamura-mdsol]

The fix made in #17853 introduced requiring a native gem, ruby-filemagic.
Installing FileMagic library just to detect content-type is not a pleasant approach and I think marcel is the way we should go in 2024.

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@wing328 @dvacca-onfido

[dvacca-onfido]

LGTM. Thanks for improving that, much better like this. 👍