Invalid and inconsistent Content-Type header

Hello,

the tool is not using a valid `Content-Type` key for the request for a default uploader by URL. [The standard](https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/Content-Type) is `Content-Type` (both words capitalized), where the tool sends `content-type` (all lower-cased).

I think this might be come from `ajax` dependency: https://github.com/codex-team/ajax/blob/bfa9730a9a939a77abab9b733f5f0397a2374502/src/index.js#L466 but I am not sure.

There is also inconsistency, because the tool behaves up to the standars for default upload by file with sending `Content-Type`.