Currently these tests work, because we are lucky to have fetched
response body available in `load_document()` to correctly sniff the MIME
type. This is a preparation for upcoming change that makes fetching
unbuffered, which will break these test unless we explicitly set the
`Content-Type` header.
Some steps are still to be implemented, namely:
* Properly aborting the read algorithm
* Handling BinaryString type properly
* Setting error on any error
But as it stands, this is enough functionality for the basic case of
reading the contents of a blob using the FileReader API.
