Python static / instance method or function?
Since you have classes that represent the configuration of various server types, you should keep the path structure in them and uses it from them. pathlib provide an abstraction over windows and unix file paths that you can store in these classes and reuse it from FileInfo.

import pathlib

class FileInfo(object):
    def __init__(self, server_info, folder_path, file_name):
        self.server_info = server_info
        self.folder_path = folder_path
        self.file_name = file_name

    def get_file_full_path(self):
        return self.server_info.paths(self.folder_path, self.file_name)


class ServerInfo(object):
    pass

class WindowsServerInfo(ServerInfo):
    paths = pathlib.PureWindowsPath

class SFTPServerInfo(ServerInfo):
    paths = pathlib.PurePosixPath


However, if the only purpose of the FileInfo class is to retrieve a path from its arguments, then you’re better of putting this function directly into ServerInfo:

class ServerInfo(object):
    def get_file_full_path(self, folder_path, file_name):
        return self.paths(folder_path, file_name)

class WindowsServerInfo(ServerInfo):
    paths = pathlib.PureWindowsPath

class SFTPServerInfo(ServerInfo):
    paths = pathlib.PurePosixPath


And you could improve it by allowing multiple path segments:

class ServerInfo(object):
    def get_file_full_path(self, *path_segments):
        return self.paths(*path_segments)

class WindowsServerInfo(ServerInfo):
    paths = pathlib.PureWindowsPath

class SFTPServerInfo(ServerInfo):
    paths = pathlib.PurePosixPath




But it seems that you're only using Python 2, so pathlib may not be available. You can still use your '%s/%s' or '%s\\%s' style approach, but you will loose some path simplifications provided by pathlib:

class ServerInfo(object):
    def get_file_full_path(self, *path_segments):
        return self.path_delimiter.join(*path_segments)

class WindowsServerInfo(ServerInfo):
    path_delimiter = '\\'

class SFTPServerInfo(ServerInfo):
    path_delimiter = '/'


Oh, and talking about '%s/%s' or '%s\\%s', str.format is the prefered way over % formating syntax.