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
UseSend it to whoever signs.
UseSend it to whomever they name.
AvoidSend it to whomever signs.