Consider the matrix $M = \begin{pmatrix} A & B \\ 0 & D \end{pmatrix}$ where $A$ and $D$ are square matrices. If $A = \begin{pmatrix} 1 & 2 \\ 3 & 4 \end{pmatrix}$ and $D = \begin{pmatrix} 5 & 6 \\ 7 & 8 \end{pmatrix}$, what is $\det(M)$?