The ornamental design for a trailer with flatbed, as shown and described.
FIG. 1 is a rear left side perspective view of a trailer with flatbed showing my new design;
FIG. 2 is a front right side perspective view thereof;
FIG. 3 is a left side elevation view thereof;
FIG. 4 is a right side elevation view thereof;
FIG. 5 is a top plan view thereof;
FIG. 6 is a front elevational view thereof; and,
FIG. 7 is a rear elevational view thereof.
The broken lines shown in the figures are included for the purpose of illustrating portions of the trailer with flatbed and the environment that form no part of the claimed design.