To isolate the factors that control the structure of nanocomposite thin films, we develop a computational model and scaling theory to investigate the behavior of diblock/nanoparticle mixtures that are confined between two hard walls. We find that in such restricted geometries a polymer-induced depletion attraction drives the particles to these walls. If the particles are chemically distinct from the walls, they will effectively modify the chemical nature of these substrates. This change in chemistry, in turn, affects the polymer-wall interactions and consequently the structure of the film. We illustrate this point by considering mixtures of particles and symmetric diblocks and show that the confining walls can be exploited to promote the self-assembly of the system into particle nanowires that extend throughout the films and are separated by nanoscale stripes of polymer domains. Such films constitute vital components in the fabrication of nanoscale devices. Furthermore, the results point to a novel technique for modifying the chemical nature of coatings and films entirely through self-assembly. Since this technique relies on entropic effects, it constitutes a fairly robust method that can be applied more generally than approaches that rely primarily on chemistry-specific enthalpic effects.