Make sure that the MIME Type of the font you are trying to upload is correct by doing the following
If you upload a file with the “debug output” checkbox in the plugin settings temporarily, the MIME-type recognized by WordPress will be displayed(turn it off after check the mime type). For example, if you have ttf font files on hand, it will say “font/sfnt”, and for otf, it will say “application/vnd.ms-opentype”.
In these cases
otf = application/vnd.ms-opentype
ttf = font/sfnt
Alternatively, if you want to ignore WordPress’ MIME type checking, disable the setting “Enable the attempt to determine the real file type of a file by WordPress core”.
