Via the Steam Store:
Various improvements to how certain particle effects are stored. One impact this has is a format change for the particle snapshot system. While this specific feature has never been officially supported, if there are any Custom Games using models generating custom snapshots we recommend that they be updated to match the new requirements (specifically, output files must always be specified and have the form “particles/*.vsnap”). We’ve also implemented a feature to automatically convert any particles referencing Valve snapshots to the new system.