Support gifs in plot viewer

Is your feature request related to a problem? Please describe.
I'm generating gifs of plots, not just PNG/SVG. I'd like to see them in the same way that still plots show up.

Additional context
Example code:

library(ggplot2)
library(gganimate)
anim <- ggplot(data.frame(x=1:10, y=rnorm(10)), aes(x=x, y=y)) +
            geom_point() +
            transition_time(x)
animate(anim)

I'm using gifski renderer. When I run this code, I get what looks like viewer trying to view a binary file:

I can see that the gif was successfully rendered into the R temporary directory and if I select it, the animation does show up correctly in VSCode.

I've tried using httpdg plot viewer and httpdg web page. httpdg supports gif, and many other formats in addition to png and svg, so not sure where the limitation is coming from.

Start at the plot viewer entry point that handles PNG and SVG output, using the reported ggplot2 and gganimate example to trace how the gifski-rendered file is detected and displayed. Done means a rendered GIF opens as an animation in the plot viewer rather than as binary content.