The ornamental design for a mobile phonefilmholder, as shown and described.
FIG. 1 is a perspective view of a mobile phone film holder showing my design;
FIG. 2 is a front view thereof;
FIG. 3 is a back view thereof;
FIG. 4 is a left side view thereof;
FIG. 5 is a right side view thereof;
FIG. 6 is a top view thereof; and,
FIG. 7 is a bottom view thereof.
The broken lines shown in the drawings depict portions of the mobile phone film holder in which the design is embodied that form no part of the claimed design.