how to send csrf token through ajax call in laravel?

i have a form with method=post.In that form i have an image upload field which is ajax.when the ajax call process,verify csrf token mismatch error occure.help me.this is my code..,

<script>
            $(document).ready(function(){
                $(document).on("click", "#upload", function() {
                    var file_data = $("#groupe_img").prop("files")[0];   // Getting the properties of file from file field
                    var form_data = new FormData();                  // Creating object of FormData class
                    form_data.append("file", file_data)              // Appending parameter named file with properties of file_field to form_data
                    form_data.append("csrftoken",document.mainform.csrftoken.value;)                 // Adding extra parameters to form_data
                    $.ajax({
                                url: "/upload_avatar",
                                dataType: 'script',
                                cache: false,
                                contentType: false,
                                processData: false,
                                data: form_data,                         // Setting the data attribute of ajax with file_data
                                type: 'post'
                       })
                })
            });
        </script>

this is my html portion

<input type="file" name="groupe_img" id="groupe_img">
<button id="upload" value="Upload">upload image</button>

tysm

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