Invalid and inconsistent Content-Type header

[zdebra]

Hello,

the tool is not using a valid Content-Type key for the request for a default uploader by URL. The standard 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.