To write a Java extension to FTP with AI2 you will need some Java code. Here is a link to what might be an example of the required Java code.
http://stackoverflow.com/questions/6527664/how-do-you-upload-a-file-to-an-ftp-server . Use it as a beginning but you will have to modify it to work with AI2.. To build YOUR extension, you will need a basic knowledge of how to program using Java., perhaps the suggestions here:
https://www.google.com/search?q=How+to+program+in+java&oq=How+to+program+in+java&aqs=chrome..69i57.8159j0j7&sourceid=chrome&ie=UTF-8
To write the Java source code you will need a Java compiler like example Eclipse or Android Studio 2 or IntelliJ IDEA . These are free compilers.
Need more help? Possibly
Creating an extension is not trivial for someone who does not have any experience programming in Java. Here is the MIT 'tutorial' showing how to build an extension:
http://ai2.appinventor.mit.edu/reference/other/extensions.html If it were EASY to build a useful aix, there would be 100's of aix extensions commercially and/or free. The MIT tutorial is a good beginning but it is not highly detailed.
Can you build your extension? Of course, but you may have to teach yourself some non-AI2 skills first.
Why build an extension yourself? When MIT makes the next AI2 upgrade, the extensions you have from other sources will probably not work with the latest versions of AI2. If the author of the extension you are using does not upgrade the extension, you will probably not be able to use that aix extension in your projects going forward.
When you build your extension, consider creating a simple tutorial showing how you built the and posting it here in the forum in the area for Extensions. Hmmm you could keep the code 'secret' but what you show might inspire other non-Java programmers to build different extensions which they might share with the community.
Good luck.
Regards,
Steve