The SEGGER FTP Server is a free and complete application. It makes it possible to easily evaluate the emFTPServer core.
The Host applications (available for Windows, Linux and macOS) use the exact same "engine", so the exact same emFTPServer code available for embedded applications. This way they are guaranteed to behave the same when run on a host as when run on an embedded target.
You are more than welcome to use Wireshark to sniff the communication between the server and any FTP client. emFTPServer is compatible with any RFC compliant FPC client, from browsers such as Firefox or Chrome to dedicated FTP Clients such as FileZilla down to simple command line clients such as ftp clients that come with most host operating systems. The provided utilities are for both evaluation as well as "productive use", meaning you are free to use them if you find them useful.