Hello. Ich would like to know if there is a better example for adding files into a activity than here  https://www.openproject.org/docs/api/endpoints/attachments/. I dont really understand where to insert the file.

Hello, 

Here's how to add attachments to work packages: https://www.openproject.org/docs/user-guide/work-packages/create-work-package/#add-attachments-to-work-packages

Best regards
Samantha