Shared/downloaded puctures don't get any file extension

#### General information

* **App version:** 1.3.2
* **System:** Android 7.1.2 LOS 14.1 galaxy s4
* **Pod:** nerdpol.ch

#### Description
When i try to share or download a picture from stream. It doesnt get any file extension.

This leads to a not working sharing at all. Cause apps cant handle that.

(I read they currently should all get a ".png however this doesn't seem to happen to me (also it has always been like this for me)

Reproduce the issue on the Android app using the reported share and download actions for a stream picture. Trace the picture handling path to find where the output filename is created, then verify the result with an app that depends on a file extension. Done means shared and downloaded pictures receive a usable extension and sharing works as expected.

Written by the indexing model from the issue text.