How to Fix \"Request Header Fields Not Allowed\" Errors in Cross-Origin File Transfers?

Resolving "Request Header Fields Not Allowed" Errors in Cross-Origin File Transfers

When attempting to send files via a POST request, developers may encounter the error: "Request header field Content-Type is not allowed by Access-Control-Allow-Headers." This error indicates that the server does not allow the specified header field.

The initial attempt to resolve this error by adding various headers, including "Access-Control-Allow-Origin" and "Access-Control-Allow-Methods," resulted in a new error: "Request header field Access-Control-Allow-Origin is not allowed by Access-Control-Allow-Headers."

Investigating further, it was discovered that the jQuery documentation warns against setting the content type to anything other than "application/x-www-form-urlencoded," "multipart/form-data," or "text/plain" for cross-domain requests. Angular's default content type is "application/json," which triggers a preflight OPTIONS request to the server.

To resolve this issue, you can either overwrite Angular's default header or allow Access-Control-Allow-Headers on the server end. Here is an example of overriding Angular's default header:

$http.post(url, data, {
    headers : {
        'Content-Type' : 'application/x-www-form-urlencoded; charset=UTF-8'
    }
});

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