[attachment-viewer] SVGs show up as big empty whitespace

[Bubu]

I guess the attachement viewer doesn't really support displaying SVGs, it still tries to though and just displays an empty space in the timeline and an empty fullscreen view.

[Bubu]

This is still happening, here's a test file: https://bubu1.eu/liberapay_donate_button.svg

[HarHarLinks]

This is still happening and really confusing. There is a big empty space both in the timeline (preview which is png is not used) nor does "opening" the image work. Please at least add an error message.

[upward4]

I can reproduce with v1.6.8