Any officer of the customs service designated by the Secretary of the Treasury for foreign service, shall, through the Department of State, be regularly and officially attached to the diplomatic missions of the United States in the countries in which they are to be stationed, and when such officers are assigned to countries in which there are no diplomatic missions of the United States, appropriate recognition and standing with full facilities for discharging their official duties shall be arranged by the Department of State. The Secretary of State may reject the name of any such officer whose assignment to the foreign post for which he has been designated would, in his judgment, be prejudicial to the public policy of the United States. The appointment of such customs officers shall be made pursuant to the civil service laws and regulations upon the nomination of the principal officer in charge of the office to which such appointments are to be made.