The OPTIONS request is a normal request: this is used to ask for permissions relative to CORS restrictions. Have a look to this page to understand how CORS work under the hood.

In your case, this is a pure CORS related issue. The OPTIONS request contains this header:

Access-Control-Request-Headers: authorization,cache-control,x-requested-with

Which means: can I use authorization, cache-control and x-requested-with headers in my cross-domain AJAX request ?

The response you get is the following:

Access-Control-Allow-Headers :Authorization, Content-Type, Accept, X-Mashape-Authorization

Which means: you're allowed to use those headers only: Authorization, Content-Type, Accept, and X-Mashape-Authorization.

As you can see, cache-control and x-requested-with are not listed in the allowed list, causing the browser to reject the request.

I've come to 2 test code sample which show this behavior:

Example 1 (working)

var data = new FormData();

data.append('image', 'http://placehold.it/300x500');



var xhr = new XMLHttpRequest();

xhr.open('POST', 'https://api.imgur.com/3/upload', true);

xhr.setRequestHeader('Authorization', 'Client-ID xxxxxxxxxx');

xhr.send(data);

Here are the preflight request's headers sent when running this code (as shown by Firefox 30 devtools, and I've removed unrelated headers such as User-Agent, Accept ...):

• OPTIONS https://api.imgur.com/3/upload


• Host: api.imgur.com


• Origin: http://local.host:8080


• Access-Control-Request-Method: POST


• Access-Control-Request-Headers: authorization


• Cache-Control: no-cache

And the corresponding response's headers

• access-control-allow-origin :*


• Access-Control-Allow-Methods :GET, PUT, POST, DELETE, OPTIONS


• Access-Control-Allow-Headers :Authorization, Content-Type, Accept, X-Mashape-Authorization

Here, we can see that we prompt access to the authorization header, and the server is accepting this header, allong with the POST method and any origin URL, so the CORS requirements are satisfied and the request is allowed by the browser.

Example 2 (not working)

var data = new FormData();

data.append('image', 'http://placehold.it/300x500');



var xhr = new XMLHttpRequest();

xhr.open('POST', 'https://api.imgur.com/3/upload', true);

xhr.setRequestHeader('Authorization', 'Client-ID xxxxxxxxxx');

// the only difference with the previous code is this line

xhr.setRequestHeader('Cache-Control', 'no-cache');

xhr.send(data);

Preflight request's headers:

• OPTIONS https://api.imgur.com/3/upload


• Host: api.imgur.com


• Origin: http://local.host:8080


• Access-Control-Request-Method: POST


• Access-Control-Request-Headers: authorization,cache-control


• Cache-Control: no-cache

Preflight response's headers (which is the same as in example 1):

• access-control-allow-origin :*


• Access-Control-Allow-Methods :GET, PUT, POST, DELETE, OPTIONS


• Access-Control-Allow-Headers :Authorization, Content-Type, Accept, X-Mashape-Authorization

Here, the Access-Control-Request-Headers header prompt access for cache-control, which the server does not provide, so the CORS requirements are not satisfied and the request is rejected by the browser.

Here's a JSFiddle referencing different working and not working demos for your problem: http://jsfiddle.net/pomeh/Lfajnebh/. Pay attention to details to understand what's going on, there is few comments but they are here to emphasis trickiest parts of the code.

As a bonus, I've sent a pull request to DropZone's GitHub repository to fix this problem (https://github.com/enyo/dropzone/pull/685) which allows you to remove pref-defined headers by DropZone. Give it a try:

var myDropzone = new Dropzone('.dropzone', {

    //...

    headers: {

        'Authorization': authorizationHeader,

        // remove Cache-Control and X-Requested-With

        // to be sent along with the request

        'Cache-Control': null,

        'X-Requested-With': null

    }

});

The code above should work with my patched version (https://github.com/pomeh/dropzone/commit/f0063db6e5697888582421865840258dec1ffdc1), whereas the code above should not:

var myDropzone = new Dropzone('.dropzone', {

    //...

    headers: {

        'Authorization': authorizationHeader,

        // remove Cache-Control and X-Requested-With

        // to be sent along with the request

    }

});