ssh_sftpd - Specifies the channel process to handle an SFTP
    subsystem.
Specifies a channel process to handle an SFTP subsystem.
subsystem_spec(Options) -> subsystem_spec()
  
Types:
Options = [{Option, Value}]
 
Is to be used together with ssh:daemon/[1,2,3]
Options:
  - {cwd, String}:
- Sets the initial current working directory for the server.
- {file_handler, CallbackModule}:
- Determines which module to call for accessing the file server. The default
      value is ssh_sftpd_file, which uses the file and
      filelib APIs to access the standard OTP file server. This option
      can be used to plug in other file servers.
- {max_files, Integer}:
- The default value is 0, which means that there is no upper limit.
      If supplied, the number of filenames returned to the SFTP client per
      READDIR request is limited to at most the given value.
- {root, String}:
- Sets the SFTP root directory. Then the user cannot see any files above
      this root. If, for example, the root directory is set to /tmp, then
      the user sees this directory as /. If the user then writes cd
      /etc, the user moves to /tmp/etc.
- {sftpd_vsn, integer()}:
- Sets the SFTP version to use. Defaults to 5. Version 6 is under
      development and limited.