We came across an article in the context of photographic images, but it could apply to your documents and personal data too.
It seems that WeTransfer have updated their terms so that using the platform gives them a royalty-free license to use the content for the purposes of operating, developing and improving the service.
That does sound rather vague, and potentially might include the training of AI, and internal data modelling, which ought to be opt-in.
This is wider than one provider though, and the monetisation of personal data seems to be accelerating.
We encourage you to check the provider(s) that you use, to make sure you are happy with their terms.