Comment by dopple

This is exactly how I do it in KDE:

  > cat ~/.local/share/kio/servicemenus/convertHEIF.desktop 
  [Desktop Entry]
  Type=Service
  MimeType=image/heif;
  Actions=convertHEIF

  [Desktop Action convertHEIF]
  Name=Convert to JPEG
  Icon=image-bmp
  Exec=/bin/sh -c "heif-dec -q 95 %u $(dirname %u)/$(basename %u .heic).jpg"