Those docs document the widget itself, which is used for multiple scenarios. Genesys Cloud does not support file uploads for web chat v2. You can request new features and share your use case at https://genesyscloud.ideas.aha.io/.
I also did that research on whether there was already an Idea that implemented the ability to add files to the chat and from what I see it is delivered.
I think it is an error on the idea (DIG-I-74). It is not implemented/available in webchat (I am 100% sure).
This other same idea is in Community Review: https://genesyscloud.ideas.aha.io/ideas/DIG-I-1047
I've spoken with the PM for this and that idea was marked as delivered in error; it was accidentally marked when this feature was delivered for web messaging. The idea has been updated with a note from the PM:
Image attachments are available via web messaging. No plans to add this functionality to web chat. This idea was updated as delivered in error, fixing to reflect the correct status.