A file with .sfd (Spline Font Database) extension is native ASCII font format for font editing software - FontForge. The software supports supports many other common font formats and is freely available. The software can be used for almost all modern operating systems including Linux, Windows, and MacOS.
SFD File Format
SFD files are ASCII files that are human readable and are easily transferred across the internet. It is identified by application/vnd.font-fontforget-sfd as its MIME type.
A common SFD file header is as shown in the following exmaple.