Images in tensorboard are not recognized as images in non chromium-based browsers.

[elimohl]

No View Image, Copy Image, Copy Image Location items in context menu in Firefox and Palemoon.

• TensorBoard version 1.6.0
• TensorFlow version 1.6.0
• Python version 3.6.4
• Browsers Mozilla Firefox 62.0.3, Moonchild Productions Pale Moon 28.1.0