Jason.DecodeError exception

I came across something that I can’t wrap my head around while building some jQuery forms on a legacy site that are submitting to our new Phoenix-driven API.

The relevant part of the jQuery submit is this:

$.ajax({
    type        : 'POST', 
    url         : url, // <-- submits to our Phoenix endpoint
    data        : {foo: "bar"}, // sample data
    contentType: 'application/json; charset=UTF-8', // <-- Causes errors!
    dataType    : 'json'
})

What I noticed is that when the contentType is explicitly specified, Phoenix gets an error:

a Jason.DecodeError exception was raised with message "unexpected byte at position 0: 0x70 ('p')

The route is using the json pipeline:

pipeline :api do
    plug(:accepts, ["json"])
end

but it only seems to work when the request is sent using the content-type application/x-www-form-urlencoded; charset=UTF-8 header.

I feel like I must be doing something stupid here, but I can’t see it. Can anyone set me straight? Thanks!