This is not a feature that is widely used.
URLs that identify files on FTP servers have a special feature that allows you to also tell the client (curl in this case) which file type the resource is. This is because FTP is a little special and can change mode for a transfer and thus handle the file differently than if it would use another mode.
You tell curl that the FTP resource is an ASCII type by appending ;type=A to
the URL. Getting the foo file from the root directory of example.com using
ASCII could then be made with:
curl "ftp://example.com/foo;type=A"
curl defaults to binary transfers for FTP, but the URL format allows you to
specify the binary type with type=I:
curl "ftp://example.com/foo;type=I"
Finally, you can tell curl that the identified resource is a directory if the type you pass is D:
curl "ftp://example.com/foo;type=D"
…this can then work as an alternative format, instead of ending the path with a trailing slash as mentioned above.