WM does not support FTP. IS supports FTP. WMIACNAP.
IS FTP is not operating system dependent except that file paths are different on Unix / Linux servers, of course.
Note that IS FTP is not a general purpose FTP solution. It is only designed to be used to invoke IS Flow or Java services upon receipt of a small- to medium-sized file with a file extension representing a content-type recognized by IS.
If you need a general purpose FTP solution there are many available inclding the ones provided by the OS vendors.
If you absolutely must use IS FTP to transfer very large files, you will want to undertake the advanced difficulty task of writing a custom content handler for the content type you are processing that streams the file’s contents to disk and then invokes your service with only the name of the temporary file.