Something is wrong with MIME handling. Opening a py file tries to use the ImageViewer for some reason, and it fails. I have the PythonEditor enabled, but it just doesn't work. Even if I disable ImageViewer, Qt Creator just fails to open any py file.
Noticed the same behavior with other file types.
This worked fine with 4.7.
Generally speaking, I think the text editor and binary editor should be optional for all mime types - if you can't find a match, fall-back to text editor if the file looks like a text file, or to binary editor otherwise. And with right-click both should be available for all types of files.