The ornamental design for a dental filmholder, as shown and described.
FIG. 1 is a isometric view of a dental film holder showing our new design;
FIG. 2 is a first side view thereof;
FIG. 3 is a second side view thereof;
FIG. 4 is a front view thereof;
FIG. 5 is a rear view thereof;
FIG. 6 is a top view thereof; and,
FIG. 7 is a bottom view thereof.
The broken lines in the drawing views are included for the purpose of illustrating portions of the dental film holder that form no part of the claimed design.