Hello Davide.
I can confirm that at the present time PixInsight can not decode the RAW RAF file from the Fuji X-T100 camera (as opposed to the Fuji X100T RAF file which PixInsight can decode).
Until the RAF file type for the Fuji X-T100 is added to PixInsight's Raw Module the only workaround is to download and use Fuji's free RAW converter software RAW FILE CONVERTER EX3.0.
http://www.fujifilm.com/support/digital_cameras/software/myfinepix_studio/rfc/Use the batch convert option in Fuji's RAW FILE CONVERTER EX3.0 to convert your raw RAF files into TIFF files then import the TIFF files to PixInsight.
When using Fuji's RAW FILE CONVERTER EX3.0 be sure not to use any of the automatic image enhancement options, just convert the RAF files to 16bit TIFF with minimum changes.
Hopefully PixInsight's Raw Module will be updated to decode the RAW RAF file from the Fuji X-T100 ASAP.
William.