Unable to delete payload after pushing it to remote server using ExpressionEvaluatingRequestHandlerAdvice

I'm trying to delete the source file that has been pushed to remote server using the ExpressionEvaluatingRequestHandlerAdvice:

    @Bean
    public Advice expressionAdvice(GenericEndpointSpec<FileTransferringMessageHandler<ChannelSftp.LsEntry>> c) {
        ExpressionEvaluatingRequestHandlerAdvice advice = new ExpressionEvaluatingRequestHandlerAdvice();
        advice.setOnSuccessExpressionString("payload.delete()");
        advice.setOnFailureExpressionString("payload + ' failed to upload'");
        advice.setTrapException(true);
        return advice;
    }

in the code below:

    @Bean
    public IntegrationFlow integrationFlow() {
        return IntegrationFlows.from(fileReader(), spec -> spec.poller(Pollers.fixedDelay(1000)))
                .transform(transformer, "transform")
                .handle(
                        Sftp.outboundAdapter(sftpSessionFactory, FileExistsMode.REPLACE)
                        .remoteDirectory(sftpRemoteDirectory), 
                        c -> c.advice(expressionAdvice(c))
                )
                .get();
    }

    @Bean
    public FileReadingMessageSource fileReader() {
        FileReadingMessageSource source = new FileReadingMessageSource();
        source.setDirectory(new File(localSourceDirectory));
        return source;
    }

And my Transformer class:

@Component
public class Transformer {

    public String transform(String filePath) throws IOException {
        String content = new String(Files.readAllBytes(Paths.get(filePath)));
        return "Transformed content: " + content;
    }

}

However, when i check the source directory, the file is still there. What am I missing here? Help please.

I'm using Spring Integration 5.2.4.

Thanks in advance!

---

Here's the working code based on @ArtemBilan's answer:

    @Bean
    public Advice expressionAdvice(GenericEndpointSpec<FileTransferringMessageHandler<ChannelSftp.LsEntry>> c) {
        ExpressionEvaluatingRequestHandlerAdvice advice = new ExpressionEvaluatingRequestHandlerAdvice();
        // advice.setOnSuccessExpressionString("payload.delete()");
        advice.setOnSuccessExpressionString("headers[file_originalFile].delete()");
        advice.setOnFailureExpressionString("payload + ' failed to upload'");
        advice.setTrapException(true);
        return advice;
    }

One more time:

public String transform(String filePath) throws IOException {
    String content = new String(Files.readAllBytes(Paths.get(filePath)));
    return "Transformed content: " + content;
}

So, your .transform(transformer, "transform") produces a String not a File. This is ok for the Sftp.outboundAdapter() because it is able to transform that string into a remote file content. But what is that advice.setOnSuccessExpressionString("payload.delete()"); should do for the String object? I believe you want to delete a file, so you need to have exactly File object for that advice to work properly.

The FileReadingMessageSource populates for us a FileHeaders.ORIGINAL_FILE header. So, you can change your expression for removal into this:

headers[file_originalFile].delete()

And you should be OK.