The design consists of the visual features of the NAIL FILE shown in the drawings, whether those features are of shape, configuration, ornamentation, pattern or a combination of the same. The stippled lines form no part of the claimed design. The following is a specification, reference being had to the accompanying drawings in which:Figure 1 is a top perspective view of the NAIL FILE.Figure 2 is a bottom perspective view of the NAIL FILE shown in Figure 1.Figure 3 is a right side elevation view of the NAIL FILE shown in Figure 1.Figure 4 is a left side elevation view of the NAIL FILE shown in Figure 1.Figure 5 is a top view of the NAIL FILE shown in Figure 1.Figure 6 is a bottom of the NAIL FILE shown in Figure 1.Figure 7 is a back view of the NAIL FILE shown in Figure 1.Figure 8 is a front view of the NAIL FILE shown in Figure 1.Figure 9 is a top perspective view of a first variant of the NAIL FILE.Figure 10 is a bottom perspective view of the NAIL FILE shown in Figure 9.Figure 11 is a right side elevation view of the NAIL FILE shown in Figure 9.Figure 12 is a left side elevation view of the NAIL FILE shown in Figure 9.Figure 13 is a top view of the NAIL FILE shown in Figure 9.Figure 14 is a bottom of the NAIL FILE shown in Figure 9.Figure 15 is a back view of the NAIL FILE shown in Figure 9.Figure 16 is a front view of the NAIL FILE shown in Figure 9.Figure 17 is a top perspective view of a second variant of the NAIL FILE.Figure 18 is a bottom perspective view of the NAIL FILE shown in Figure 17.Figure 19 is a right side elevation view of the NAIL FILE shown in Figure 17.Figure 20 is a left side elevation view of the NAIL FILE shown in Figure 17.Figure 21 is a top view of the NAIL FILE shown in Figure 17.Figure 22 is a bottom of the NAIL FILE shown in Figure 17.Figure 23 is a back view of the NAIL FILE shown in Figure 17.Figure 24 is a front view of the NAIL FILE shown in Figure 17.Figure 25 is a top perspective view of a third variant of the NAIL FILE.Figure 26 is a bottom perspective view of the NAIL FILE shown in Figure 25.Figure 27 is a right side elevation view of the NAIL FILE shown in Figure 25.Figure 28 is a left side elevation view of the NAIL FILE shown in Figure 25.Figure 29 is a top view of the NAIL FILE shown in Figure 25.Figure 30 is a bottom of the NAIL FILE shown in Figure 25.Figure 31 is a back view of the NAIL FILE shown in Figure 25.Figure 32 is a front view of the NAIL FILE shown in Figure 25.