почему изменяется request

Заголовки

Request URL:https://nextcloud.ru/ocs/v1.php/apps/files_sharing/api/v1/shares
Request Method:OPTIONS
Status Code:405 Method Not Allowed
Remote Address:92.126.152.50:443
Referrer Policy:no-referrer-when-downgrade
Response Headers
Access-Control-Allow-Credentials:true
Access-Control-Allow-Headers:Authorization, Content-Type, Depth
Access-Control-Allow-Methods:PROPFIND, POST, OPTIONS, GET, PUT, DELETE, MKCOL, PROPPATCH, COPY, MOVE, LOCK, UNLOCK
Access-Control-Allow-Origin:https://m.mysite.ru
Cache-Control:no-store, no-cache, must-revalidate
Connection:Keep-Alive
Content-Language:ru
Content-Length:0
Content-Security-Policy:default-src 'self'; script-src 'self' 'unsafe-eval' 'nonce-N3VMT3RJWmZ0V1U3V2hTR3AyVTFjYmJ1SDhXNWpDdk9DcVJUcGJtZENIdz06Z2EzM2djSTcvd0JqRkNQdzhpRlRBWUd2SnJiTnkzNm5YdE5xOE43VlFqND0='; style-src 'self' 'unsafe-inline'; frame-src *; img-src * data: blob:; font-src 'self' data:; media-src *; connect-src *
Content-Type:text/xml; charset=UTF-8
Date:Tue, 26 Sep 2017 02:23:40 GMT
Expires:Thu, 19 Nov 1981 08:52:00 GMT
Keep-Alive:timeout=5, max=98
Pragma:no-cache
Server:Apache/2.4.27 (FreeBSD) PHP/7.1.9 Phusion_Passenger/5.1.8 OpenSSL/1.0.2k-freebsd
X-Content-Type-Options:nosniff
X-Download-Options:noopen
X-Permitted-Cross-Domain-Policies:none
X-Powered-By:PHP/7.1.9
X-Robots-Tag:none
X-XSS-Protection:1; mode=block
Request Headers
Accept:*/*
Accept-Encoding:gzip, deflate, br
Accept-Language:ru-RU,ru;q=0.8,en-US;q=0.6,en;q=0.4
Access-Control-Request-Headers:authorization,content-type,ocs-apirequest,request-method
Access-Control-Request-Method:POST
Connection:keep-alive
Host:penguincloud.ru
Origin:https://m.mysite.ru
Referer:https://m.mysite.ru/
User-Agent:Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/61.0.3163.100 Safari/537.36

Код JS

    $.ajax({
        type: 'POST',
        url: 'https://nextcloud/ocs/v1.php/apps/files_sharing/api/v1/shares',
        data: {path: item._itemDiv.classList[0], shareType: 3, publicUpload: false, permissions: 1},
        success: successCallback,
        error: errorCallback,
        beforeSend: function (xhr) {
            xhr.setRequestHeader('Authorization', 'Basic '+btoa(settings.username+':'+settings.password));
            xhr.setRequestHeader('Content-Type', 'application/xml');
            xhr.setRequestHeader('OCS-APIRequest', true);
        }
    });

ВОПРОС

Почему в первом(верхнем) разделе заголовка в качестве метода становится метод OPTION, ведь в запросе я явно указываю метод POST?