When I have an app.net url like https://photos.app.net/5269262/1 - how can I retrieve the image thumbnail of the post?
Running a curl on above url shows a redirect
bash-3.2$ curl -i https://photos.app.net/5269262/1
HTTP/1.1 301 MOVED PERMANENTLY
Location: https://alpha.app.net/pfleidi/post/5269262/photo/1
Following this gives a html page that contains the image in a form of
img src='https://files.app.net/1/60621/aWBTKTYxzYZTqnkESkwx475u_ShTwEOiezzBjM3-ZzVBjq_6rzno42oMw9LxS5VH0WQEgoxWegIDKJo0eRDAc-uwTcOTaGYobfqx19vMOOMiyh2M3IMe6sDNkcQWPZPeE0PjIve4Vy0YFCM8MsHWbYYA2DFNKMdyNUnwmB2KuECjHqe0-Y9_ODD1pnFSOsOjH' data-full-width='2048' data-full-height='1536'
Inside a larger block of <div>tags.
The files api in app.net allows to retrieve thumbnails but I somehow don't get the link between those endpoints and above urls.
The photos.app.net is just a simple redirecter. It is not part of the API proper. In order to get the thumbnail, you will need to fetch the file directly using the file fetch endpoint and the file id (http://developers.app.net/docs/resources/file/lookup/#retrieve-a-file) or fetch the post that the file is included in and examine the oembed annotation.
In this case, you are talking about post id 5269262 and the URL to fetch that post with the annotation is https://alpha-api.app.net/stream/0/posts/5269262?include_annotations=1 and if you examine the resulting json document you will see the thumbnail_url.