Tor Browser only render HTML for local pages via file://, no images/CSS

In Tor Browser 7.0.10 (and earlier too, see below), if I open a local page via file:// only the HTML is rendered, but images are broken and CSS isn't applied at all. For Tails this breaks several important features, like reading the local copy of the documentation (e.g. when offline) and the start page for our Unsafe Browser.

This breakage started earlier, and it is a bit "interesting":

• Tor Browser 7.0.7 is the last ok release (both images and CSS loads).

• Tor Browser 7.0.7 -> 7.0.8 only upgrades Torbutton 1.9.7.8 -> 1.9.7.9 which only updates some translations + fixups on the donation banner. That is somehow enough to break images on local pages (but CSS is still fine). Disabling Torbutton makes the images work again.

• Tor Browser 7.0.8 -> 7.0.9 only fixes #24052 (moved) ("Streamline handling of file:// resources") which breaks both images and CSS. I say "both" despite the previous bullet indicating that Torbutton is responsible for breaking images, because disabling Torbutton no longer fixes image loading in this version. So it indeed seems that the fix for #24052 (moved) alone breaks both CSS and images.