1. A method, comprising: at a computing device comprising local storage, one or more processors, and storage media storing one or more computer programs executed by the one or more processors to perform the method, performing operations comprising: detecting an attempt to access a placeholder file corresponding to a content item, the placeholder file stored in the local storage of the computing device; determining one or more identifiers of one or more content item blocks of which the content item is composed; determining any content item blocks of the one or more content item blocks that are not stored in the local storage of the computing device; downloading, from one or more other computing devices coupled to a network to which the computing device is coupled, any content item blocks of the one of more content item blocks that are not stored locally in the local storage of the computing device; and filling the placeholder file with the one or more content item blocks.