src/Uploadcare/File.php
public function op($operation)
    {
        if (!$operation) {
            return $this;
        }
        $result = clone $this;
        $result->operations[]['custom'] = $operation;
        return $result;
    }
public function op($operation)
    {
        if (!$operation) {
            return $this;
        }
        $result = clone $this;
        $result->operations[]['custom'] = $operation;
        return $result;
    }
Add any custom operation.

@param string $operation
@return File