Yes, you can enable end-user document upload for both chatbots and smart-forms. These uploads do not use credits nor do they count against your knowledge base limit. However, I believe end-users cannot upload gigantic documents. The 100mb limit applies to them. For text-based files this is fairly massive.
This text is not customizable. You bring up a great point though! We just added vision models to the platform, so have overlooked that piece of copy in the end-user upload field in Smart Forms. Thanks for pointing this out. We will update that copy.