You are trying to create a testimonial input form with ninja forms so that people can upload their testimonial and logo and it gets published on a page, correct?
File uploads is a ninja forms paid extension.
Let me suggest something different.
You can achieve this using a simple and free testimonial plugin that is compatible with CP.
These three free plugins are just examples and are free and compatible with CP. All of them support the input of a customer logo, a feedback or comment and other details customers might want to input in their feedback.
You can put the form that the testimonial plugin comes with on a page to let your customers submit their feedback on their own, and show the carousel of their logos in your homepage.