This article explains the Attached Files block that can be added to design an app.
Please note that this block is different from the File attachment or image upload block. This allows end-users to download a set fixed file attached by the app designers.
Click on the block and start editing on the panel that appears on the right side.
This is what appears as the name of the attached file to the end-user.
This is the text that goes below the attached file to help the user.
Click on the upload icon to browse a file and attach to the app.
Which block should I use if I want the end-users to upload files or capture images?
You should use the Camera, image, and Files block for such a purpose.