From Synfig Studio :: Documentation
About File Formats
Synfigs native working file format is sif, an uncompressed human-readable XML file format. The default format is sifz which is a sif file compressed in a gzip container. As of Synfig Studio 1.0 there is a new file format, sfg, which is a sif along with all assets it uses; images, audio, etc compressed into a zip container.
To use any of the formats just add the file suffix to your file name while saving in Synfig.