For config options like files.image-mime-types, when edited from the web UI the correct value is something like:

example/a
example/b

However, the existing values are shown in JSON:

{
  "example/a" : true,
  "example/b" : true
}

If you copy/paste the JSON, you end up with a completely bogus configuration.

We should probably show the current value in the same format, and possibly provide examples. This is sort of hard to validate because strings can contain any characters other than newlines.