The ornamental design for a postmountingdevice, as shown and described.
FIG. 1 is a front perspective view of a post mounting device showing my new design, wherein the features shown in broken lines depict environmental subject matter only and form no part of the claimed design;
FIG. 2 is a front elevational view of the same, wherein the features shown in broken lines depict environmental subject matter only and form no part of the claimed design;
FIG. 3 is a back elevational view of the same, wherein the features shown in broken lines depict environmental subject matter only and form no part of the claimed design;
FIG. 4 is a right side elevational view of the same, wherein the features shown in broken lines depict environmental subject matter only and form no part of the claimed design;
FIG. 5 is a left side elevational view of the same;
FIG. 6 is a top plan view of the same;
FIG. 7 is a bottom plan view of the same, wherein the features shown in broken lines depict environmental subject matter only and form no part of the claimed design; and,
FIG. 8 is a front perspective view of the post mounting device, wherein the features shown in broken lines depict environmental subject matter only and form no part of the claimed design.