This function lets the user download a file from Dropbox, specifying its name and using a previously created token.
db_upload(filename, dir, delete_file = FALSE, token_dir = NA,
token_name = "token_pers.rds")
String. File's name
String. Directory you wish to upload the file to
Boolean. Delete local file after uploading?
Character. Credential's local directory
Character. RDS file name with your token's data