这是我能够找到的解决方案，实际上我看到了很多常见的错误。

JS中没有正确附加文件dataFilter: function() 可以替换为dataType: \'json\'.可替换为event.stopPropagation() &；e.preventDefault().

HTML

<form id="form_import" name="form_import" method="post" enctype="multipart/form-data" action >
    <input id="file_import" name="import_data" type="file" />
    <br/>
    <button id="btn_import" type="submit" >Import</button>
</form>

jQuery(document).ready(function($){
    // PHP files could echo an Nonce. This however localizes scripts.
    var importNonce = localizedData.import_nonce;

    $(\'#form_import\').submit( function( event ) {
        event.stopPropagation(); // Stop stuff happening
        event.preventDefault(); // Totally stop stuff happening

        var formData = new FormData();
        var importFiles = $(\'#file_import\')[0].files;

        // For EACH file, append to formData.
        // NOTE: Just appending all of importFiles doesn\'t transition well to PHP.
        jQuery.each( importFiles, function( index, value ) {
            var name = \'file_\' + index;
            formData.append( name, value )
        });

        formData.append( \'action\', \'ajax_file_import\' );
        formData.append( \'_ajax_nonce\', importNonce );

        jQuery.ajax({
            url: ajaxurl,
            type: \'POST\',
            data: formData,
            cache: false,
            dataType: \'json\', // This replaces dataFilter: function() && JSON.parse( data ).
            processData: false, // Don\'t process the files
            contentType: false, // Set content type to false as jQuery will tell the server its a query string request


            beforeSend: function( jqXHR, settings ){
                // (OPTIONAL) Alt. AJAX concept.
                // Removes the old IFrame if any.
                var element = document.getElementById(\'import_IF\');
                if ( element !== null ){
                    element.parentNode.removeChild( element );
                }
                // END (OPTIONAL).
            },
            success: function( data, textStatus, jqXHR ) {
                console.log( \'Return from PHP AJAX function/method.\' );

                // Do stuff with data.values

                // (OPTIONAL) Alt. AJAX concept.
                // Required for downloading in AJAX.
                var paramStr = \'\';
                paramStr += \'?_ajax_nonce=\' + data._ajax_nonce;
                paramStr += \'&action=\'      + data.action;
                paramStr += \'&filename=\'    + data.filename;

                var elemIF = document.createElement("iframe");
                elemIF.id = \'import_IF\'
                elemIF.style.display = "none";
                elemIF.src = ajaxurl + paramStr;

                document.body.appendChild(elemIF);
                elemIF.parentNode.removeChild(elemIF);
                // END (OPTIONAL).
            },
            complete: function(jqXHR, textStatus){
                console.log( \'AJAX is complete.\' );

                // (OPTIONAL) Alt. AJAX concept.
                // Clean up IFrame.
                var element = document.getElementById(\'import_IF\');
                if ( element !== null ){
                    element.parentNode.removeChild( element );
                }
                // END (OPTIONAL).
            }
        });// End AJAX.
    });// End .submit().
});

<?php
class server {
    public function __construct() {
        // Example for adding script.
        add_action( \'admin_enqueue_scripts\', array( $this, \'enqueue_example\' ) );
        add_action( \'wp_ajax_ajax_file_import\', array( $this, \'ajax_import_example\' ) );
    }

    public funtion enqueue_example() {
        wp_register_script(
            \'example-js\',
            FILE_URL,
            array(),
            VERSION,
            false
        );
        wp_enqueue_script( \'example-js\' );

        $data = array(
            \'import_nonce\'  => wp_create_nonce( \'ajax_file_import_nonce\' ),
        );

        wp_localize_script( \'example-js\', \'localizedData\', $data );
    }

    public function ajax_import_example() {
        check_ajax_referer( \'ajax_file_import_nonce\' );

        $raw_content = array();
        $i = 0;
        while ( isset( $_FILES[ \'file_\' . $i ] ) ) {
            $file_arr = $_FILES[ \'file_\' . $i ];
            $file_content = file_get_contents( $file_arr[\'tmp_name\'] );
            $raw_content[]  = json_decode( $file_content );
            $i++;
        }

        $new_data = array();
        // Do Stuff with new data.
        update_option( \'example_database\', $new_data );

        // (OPTIONAL) Add data to return to AJAX Success
        $rtn_data = array(
            \'action\'               => \'apl_import\',
            \'_ajax_nonce\'          => wp_create_nonce( \'alt_ajax\' ),
        );
        echo json_encode( $rtn_data );
        // END (OPTIONAL).

        die();
    }
}
?>