Multimedia provisioning over Wi-Fi networks is a complex task that should consider also novel issues stemming from the specific behavior of IEEE 802.11 protocols. In particular, we claim the need for visibility of context data about the IEEE 802.11 performance anomaly, i.e., the situation where even a single node located at the borders of the coverage area of a Wi-Fi access point produces a relevant degradation in the connectivity quality of all other nodes in the area. The paper proposes a novel application-level middleware that counteracts IEEE 802.11 anomaly without imposing any modification in standard Wi-Fi protocols, thus permitting to maintain the current wide base of installed equipment. Our middleware portably detects anomaly situations via decentralized standard mechanisms available at clients; anomaly awareness is used to promptly react with application-level management operations (flow quality downscaling and traffic shaping) that both preserve the goodput at nodes in well-covered areas and minimize quality degradations at clients generating the anomaly. The reported experimental results point out the feasibility of application-level middleware approaches also in the challenging multimedia area.