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;
FIG. 2 is a front elevational view of the same;
FIG. 3 is a back elevational view of the same;
FIG. 4 is a left side elevational view of the same;
FIG. 5 is a right side elevational view of the same;
FIG. 6 is a top view of the same;
FIG. 7 is a bottom view of the same; and,
FIG. 8 is a reduced scale, environmental, front perspective view thereof.
The broken line showing of a post and simulative fan decoration is included for the purpose of illustrating environment and forms no part of the claimed design.