What is the ruling on working for a file-sharing website where the payment depends on reaching a certain number of downloads, and is this earning lawful or unlawful?

It is permissible to contract for a fee (ju'l) in exchange for a specific number of uploads or files uploaded, provided that the website's activity is permissible and what is uploaded is lawful. Ju'alah is the allocation of a known sum of money to someone who performs a permissible act, whether known or unknown. One must scrutinize the content of the files due to the widespread use of websites for publishing harmful or Islamically impermissible files.