  In [C. E. Kenig and E. M. Stein, Multilinear estimates and fractional
integration, Math. Res. Lett., 6(1):1-15, 1999], the following type of
multilinear fractional integral \[
  \int_{\mathbb{R}^{mn}} \frac{f_1(l_1(x_1,\ldots,x_m,x))\cdots
f_{m+1}(l_{m+1}(x_1,\ldots,x_m,x))}{(|x_1|+\ldots+|x_m|)^{\lambda}} dx_1\ldots
dx_m \] was studied, where $l_i$ are linear maps from $\mathbb{R}^{(m+1)n}$ to
$\mathbb{R}^n$ satisfying certain conditions. They proved the boundedness of
such multilinear fractional integral from $L^{p_1}\times \ldots \times
L^{p_{m+1}}$ to $L^q$ when the indices satisfy the homogeneity condition. In
this paper, we show that the above multilinear fractional integral extends to a
linear operator for functions in the mixed-norm Lebesgue space $L^{\vec p}$
which contains $L^{p_1}\times \ldots \times L^{p_{m+1}}$ as a subset. Under
less restrictions on the linear maps $l_i$, we give a complete characterization
of the indices $\vec p$, $q$ and $\lambda$ for which such an operator is
bounded from $L^{\vec p}$ to $L^q$. And for $m=1$ or $n=1$, we give necessary
and sufficient conditions on $(l_1, \ldots, l_{m+1})$, $\vec p=(p_1,\ldots,
p_{m+1})$, $q$ and $\lambda$ such that the operator is bounded.
