The fragment of the file name that becomes part of the file names of the uploaded video files in their respective upload destinations (see setting
output). That is, the full name of the file also includes a timestamp, a random token, and (if available) the original name of the input video file. Note that the sequence given in
upload.filename will be sanitized to use only these characters, digits, the underscore ("_"), and the hyphen (-) symbol. Other characters will be replaced with the underscore.