We are working to move away from using decal extensions for DLC; and instead use the built-in feature in FGU that allows any image to be used as a background decal.
Decals accept either png or webp formatted images. webp formatted images will be considerably smaller and our recommendation is to use these.
Files must be formatted with _decal appended to the end of the filename. Example: Filename1_decal.webp Filename2_decal.webp Filename3_decal.webp
This will allow them to be searchable within the assets window.
It is recommended that filenames include terms that describe the source and content. Example: LMOP_cover_decal.webp MonsterManual_beholder_decal.webp MonsterManual_scarecrow_decal.webp OOTA_demogorgon_decal.webp OOTA_underdark_landscape_decal.webp PHB_cover_decal.webp VGM_altcover_decal.webp VGM_beholder_decal.webp VGM_cloudwyrm_decal.webp VGM_gnollpack_decal.webp VGM_gnolls_decal.webp VGM_tavern_decal.webp VGM_tome_decal.webp
Create an images/Decals/ folder and place all decal files in it.
This is a Fantasy Grounds Unity-only feature. This will not work in Fantasy Grounds Classic.