Issue with an App Part

Greetings,

Using REST and JavaScript, I have developped an app which can upload a document to SharePoint. This app works fine.

But I needed it as a WebPart Component, so I added this to my project in Visual Studio and I configured it. Finally, I inserted my app as an "App Part" to a page in SharePoint. At first sight, all fields look good but I cannot upload my file. I am getting the error "Forbidden" whereas I have all permissions in the collection of sites.

Thanks in advance for your answer.

Mike