For binary response, HAR allows base64 encoding so data is untouched.
But no such thing exists for binary posted data. Some generators write JSON
with the binary posted data as if it were latin-1 encoded text.
But when reading, what can we do? Think it's binary to encode back to latin-1?
Or think it's text?
HAR gives no answer, so we add a non-standard "x-binary" key to indicate
"text" is actually binary disguised with latin-1.