I tested it on Android Lollipop (version 5.0.2) and Stack Exchange app version 1.0.92
Steps to recreate the scenario:
I tried to upload a
.png file and it was uploaded without an issue. Renaming it to a
.jpeg file also didn't cause a problem.
However if, for example, I've a file which is named
mypetdog.jpg and I rename it to just
mypetdog (without any filename extension) and then try to upload it, I get a red banner at the top which says image_mime_type must be set.
Possible issue and fix:
The image you're trying to upload is perhaps missing an extension. Try adding a filename extension to your image, which basically means to change
mypetdog.jpg or something equivalent.