Steps to reproduce:
1. Use Safari for manual activation
2. Follow this guide http://docs.unity3d.com/Manual/ManualActivationGuide.html
2. On the step 7. from the guide observe that file downloaded from Safari gets .xml extension
3. When you try to upload this file to the Editor, file can not be selected
4. Manually changing the name of the file does not help - file is still not recognised by the Editor

OS: OS X 10.11
Browser: Safari 9.0.1
Editor: 5.3.0f2

I also tested with previous versions of Unity - 4.6.9 and 5.2.2 with Safari versions 7.0.6 and 9.0.1 - issue is exactly the same.

Please note, on Chrome file is downloaded and saved without .xml extension.