Given a URL string that has already been downloaded, enqueue
    necessary conversion to get to target type

def enqueue_conversion_path(url_string, to_type, enqueue_convert):
    '''
    Given a URL string that has already been downloaded, enqueue
    necessary conversion to get to target type
    '''
    target_ts = TypeString(to_type)
    foreign_res = ForeignResource(url_string)

    # Determine the file type of the foreign resource
    typed_foreign_res = foreign_res.guess_typed()

    if not typed_foreign_res.cache_exists():
        # Symlink to new location that includes typed extension
        typed_foreign_res.symlink_from(foreign_res)

    # Now find path between types
    original_ts = typed_foreign_res.typestring
    path = singletons.converter_graph.find_path(original_ts, target_ts)

    # Loop through each step in graph path and convert
    is_first = True
    for converter_class, from_ts, to_ts in path:
        converter = converter_class()
        in_resource = TypedResource(url_string, from_ts)
        if is_first:  # Ensure first resource is just the source one
            in_resource = TypedForeignResource(url_string, from_ts)
        out_resource = TypedResource(url_string, to_ts)
        enqueue_convert(converter, in_resource, out_resource)
        is_first = False