The ornamental design for a portfolio, substantially as shown and described.
FIG. 1 is a front perspective view of the portfolio.
FIG. 2 is front view of the portfolio.
FIG. 3 is back view of the portfolio.
FIG. 4 a left view of the portfolio.
FIG. 5 is a right view of the portfolio.
FIG. 6 is a top view of the portfolio.
FIG. 7 is a bottom view of the portfolio; and,
FIG. 8 is another front perspective view of the portfolio.
The broken lines immediately adjacent the shaded areas represent the bounds of the claim and all other broken lines are for illustrative purposes only. None of the broken lines form part of the claimed design.