Are you talking about the SFD (FontForge internal file format) or for the fonts generated (or both?)
Since FontForge's file format is cleartext, the MIME type could any of the following I think
text/x-fontforge
application/x-fontforge
application/vnd.fontforge-sfd
application/vnd.fontforge.sfd
Although I think one of the latter two would be prefered in modern day coding.
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