For some reason, creating a URL from a string decoded from the container
was producing URL objects that could not be round-tripped through
PropertyListEncoder/Decoder. Decoding a URL directly from the container
works correctly.
Unlike Mastodon, Pleroma includes a URL query component in attachment URLs.
The URL(lenient:) initializer did not count query parameters (e.g. '?') as valid URL characters
and incorrectly escaped them resulting in image requests that 404'd.