File.copy fails if target exits

// code placeholder
import qbs
import qbs.File
import qbs.BinaryFile

Project {
    Product {
        name: "copy"
        //type: base.concat(["archiver.input-list", "package"])
        type: "package"
        property bool enableCodeSigning: true
        files: ["code.c"]
        FileTagger {
            patterns: ["*.c"]
            fileTags: ["archiver.input-list"]
        }
        Rule {
            inputs: ["archiver.input-list"]
            Artifact {
                filePath: input.fileName + ".signed"
                fileTags: ["package"]
            }
            prepare: {
                var cmd;
                if (product.enableCodeSigning) {
                    cmd = new JavaScriptCommand();
                    cmd.description = "signing " + output.fileName;
                    cmd.sourceCode = function() {
                        var source = new BinaryFile(input.filePath, BinaryFile.ReadOnly);
                        var target = new BinaryFile(output.filePath, BinaryFile.WriteOnly);
                        target.resize(0);
                        while (true) {
                            var data = source.read(1);
                            if (!data || data.length == 0)
                                break;
                            target.write(data);
                        }
                        target.write([0xAB, 0xCD]);
                        source.close();
                        target.close();
                    }
                } else {
                    cmd = new JavaScriptCommand();
                    cmd.description = "copying without signing " + output.fileName;
                    cmd.source = input.filePath;
                    cmd.target = output.filePath;
                    cmd.sourceCode = function() {
                        //if (File.exists(target ))
                        //    File.remove(target);
                        File.copy(source, target);
                    }
                }
                return cmd;
            }
        }
    }
}