The ornamental design for a mountingpost, as shown and described.
FIG. 1 is a front-top-right perspective view of a mounting post showing my new design;
FIG. 2 is a front-plan view thereof;
FIG. 3 is a back-plan view thereof;
FIG. 4 is a left elevation view thereof;
FIG. 5 is a right elevation view thereof;
FIG. 6 is a top elevation view thereof;
FIG. 7 is a bottom elevation view thereof; and,
FIG. 8 is a front-top-right perspective view of the present invention showing my new design mounted on headwear.
The broken line showing the mounting post in FIGS. 1-8 are included for the purpose of showing environmental structure and form no part of the claimed design.