So you are already handling one endpoint in your code so you understand the details. Because you've got this one, I am confident you can make a handler for another endpoint. You can think of an endpoint consisting of a handler residing at the end of a URI. So when an HTTP client requests a specific URI, your HTTP server at that location handles the request in the expected way.
userlucasm:
Does it mean that i just can operate in one direction ... ?
No, you can definitely do this in both directions the way you have specified. We just need to add some code to your server.
userlucasm:
Does it mean ... i can't doing 2nd and 3rd step without programming an endpoint ?
So you already have an HTTP server running and have created an endpoint to handle a root request, you just don't have the code in place to handle the specific request you want in the way you have specified.
This is your handler for the root of your HTTP server. Everything that happens inside there is what your server is doing if an HTTP client (in this case a web browser such as Chrome) has requested yourserver.com/. You are using this endpoint to list the files.
if (strstr(clientline, "GET /") != 0){...}
Now, you can make another endpoint to remove the files, by creating another handler on your server. I suggest something like yourserver.com. You can make that an index that lists all the files to remove. You can then dynamically handle /remove/[filename] to remove specific filenames.
What I would do first to understand the concept would be to make a static endpoint /remove/file1.txt and have that use the SD.remove("file2.txt"); Then you can make it dynamic to handle all the files on your card the same way you did to list all the files on your card.
Note: yourserver.com can also be /directory/file.ext