extended_name
If this argument is FALSE, then the downloaded
file will have a filename in the following format:
itemidentifier.extension, e.g., thedamnationofth00133gut.txt.
If there are multiple files of the same file type for an item, then the
file names will not be unique. If this argument is TRUE, them the
downloaded file will have a filename in the following format:
itemidentifier-original-filename.extension, e.g.,
thedamnationofth00133gut-133.txt.
overwrite
If TRUE, this function will download all files and
overwrite them on disk if they have already been downloaded. If
FALSE, then if a file already exists on disk it will not be
downloaded again but other downloads will proceed normally.