what-clause: agreement
The number of the verb after a clause beginning with what.
A clause beginning with what is singular when what means the thing that: What she filed is on the desk. Make the verb plural only when what plainly means the things that, and the complement is a plural count noun: What we need are two new bolts. When the number is in doubt, keep the verb singular.
Examples
Use What he wants is a clean copy.
Use What we need are clean copies.
Avoid What she filed are on the desk. (what means the thing that)