docs: def static_file(filename, root, mimetype='auto', download=False, charset='UTF-8'):
      """ Open a file in a safe way and return :exc:`HTTPResponse` with status
          code 200, 305, 403 or 404. The ``Content-Type``, ``Content-Encoding``,
          ``Content-Length`` and ``Last-Modified`` headers are set if possible.
          Special support for ``If-Modified-Since``, ``Range`` and ``HEAD``
          requests.
  
          :param filename: Name or path of the file to send.
          :param root: Root path for file lookups. Should be an absolute directory
              path.
          :param mimetype: Defines the content-type header (default: guess from
              file extension)
          :param download: If True, ask the browser to open a `Save as...` dialog
              instead of opening the file with the associated program. You can
              specify a custom filename as a string. If not specified, the
              original filename is used (default: False).
          :param charset: The charset to use for files with a ``text/*``
              mime-type. (default: UTF-8)
      """
  
      root = os.path.abspath(root) + os.sep
      filename = os.path.abspath(os.path.join(root, filename.strip('/\\')))
      headers = dict()
  
      if not filename.startswith(root):
          return HTTPError(403, "Access denied.")
      if not os.path.exists(filename) or not os.path.isfile(filename):
          return HTTPError(404, "File does not exist.")
      if not os.access(filename, os.R_OK):
          return HTTPError(403, "You do not have permission to access this file.")
  
      if mimetype == 'auto':
          mimetype, encoding = mimetypes.guess_type(filename)
          if encoding: headers['Content-Encoding'] = encoding
  
      if mimetype:
          if mimetype[:5] == 'text/' and charset and 'charset' not in mimetype:
              mimetype += '; charset=%s' % charset
          headers['Content-Type'] = mimetype
  
      if download:
          download = os.path.basename(filename if download == True else download)
          headers['Content-Disposition'] = 'attachment; filename="%s"' % download
  
      stats = os.stat(filename)
      headers['Content-Length'] = clen = stats.st_size
      lm = time.strftime("%a, %d %b %Y %H:%M:%S GMT", time.gmtime(stats.st_mtime))
      headers['Last-Modified'] = lm
  
      ims = request.environ.get('HTTP_IF_MODIFIED_SINCE')
      if ims:
          ims = parse_date(ims.split(";")[0].strip())
      if ims is not None and ims >= int(stats.st_mtime):
          headers['Date'] = time.strftime("%a, %d %b %Y %H:%M:%S GMT", time.gmtime())
          return HTTPResponse(status=304, **headers)
  
      body = '' if request.method == 'HEAD' else open(filename, 'rb')
  
      headers["Accept-Ranges"] = "bytes"
      ranges = request.environ.get('HTTP_RANGE')
      if 'HTTP_RANGE' in request.environ:
          ranges = list(parse_range_header(request.environ['HTTP_RANGE'], clen))
          if not ranges:
              return HTTPError(416, "Requested Range Not Satisfiable")
          offset, end = ranges[0]
          headers["Content-Range"] = "bytes %d-%d/%d" % (offset, end-1, clen)
          headers["Content-Length"] = str(end-offset)
          if body: body = _file_iter_range(body, offset, end-offset)
          return HTTPResponse(body, status=206, **headers)
      return HTTPResponse(body, **headers)