whoever / whomever
The case follows the job inside the clause, even after a preposition.
Whoever is the subject form. Whomever is the object form. Judge the form by the clause itself, even if a preposition stands in front of it. Give it to whoever files first is right, because whoever is the subject of files. Give it to whomever they choose is right, because whomever is the object of choose.
Examples
Use Send it to whoever signs.
Use Send it to whomever they name.
Avoid Send it to whomever signs.