Increasingly, the source materials on which historians of computing work are either born digital or have been digitized. How these digital treasures are preserved, and how continuing access to them may be secured over the long term are therefore matters of concern to us. One of the key determinants of whether this material will remain accessible is the file format in which the source materials are stored. Choosing the "wrong" format will have significant implications for the extent to which files might be supported by systems, automated tools, or workflow associated with the digital content life cycle processes in libraries, archives, and other digital repositories. For this reason, digital preservationists have always taken a keen interest in file formats, and are understandably motivated to maintain up-to-date information on the characteristics of different formats, and the effect these have on their preservability over time. A considerable amount of digital material is now subject to mandatory deposit to national libraries, meaning even data creators who have no interest in preservation or long-term access need to develop some understanding of the formats preferred, or deemed acceptable by repositories and why.