File created by Vuze, a cross-platform BitTorrent client; contains data for a file as it is being downloaded (e.g., ".avi.az!" for an .AVI file); used for marking the file as a download in progress, and the ".az!" file extension is removed when the download is complete.
AZ! files are only supported by some versions of Vuze. For supporting versions, you can turn off the use of the ".az!" extension by selecting Tools → Options..., choosing "Files," and unchecking the "Add suffix to incomplete files" option.
NOTE: When Vuze initiates a download, it allocates the entire target file size on disk, even though the file has not yet been downloaded.