Why is my \"Access-Control-Allow-Origin\" header not allowed in \"Access-Control-Allow-Headers\" when sending files with a POST request in Angular?

Issue: Cross-Origin Request Errors with Headers

When attempting to send files with a POST request, developers may encounter an error indicating that a specific request header is not allowed by the Access-Control-Allow-Headers response header.

To resolve this issue, the developer added the following headers to the request:

"Access-Control-Allow-Origin" : "*",
"Access-Control-Allow-Methods" : "GET,POST,PUT,DELETE,OPTIONS",
"Access-Control-Allow-Headers": "Content-Type, Access-Control-Allow-Headers, Authorization, X-Requested-With"

However, this resulted in a new error stating that the "Access-Control-Allow-Origin" header is not allowed by the "Access-Control-Allow-Headers" header.

Solution: Header Restrictions and Default Content Type

For cross-origin requests, browsers will send a preflight OPTIONS request if the content type is not set to one of three specific types: "application/x-www-form-urlencoded", "multipart/form-data", or "text/plain".

By default, Angular sets the content type to "application/json", which is not one of the acceptable types for cross-origin requests. This triggers the preflight OPTIONS request, which is then rejected due to the restrictions on the "Access-Control-Allow-Headers" header.

To resolve this issue, the developer can overwrite the default content type in Angular or allow the "Access-Control-Allow-Headers" header on the server side.

An Angular sample that overwrites the default header:

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

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