json-generation: recognise .tar.(xz|gz|zst) in extract_file_extension

Currently the function falls through to the generic "${n##*.}" fallback
for tar-based image archives, returning just "xz" (or "gz"/"zst") as the
file_extension. Downstream consumers — notably armbian-router's map
loader — only add entries whose extension ends with img.xz or a
registered specialExtension, so all 3 Arduino UNO Q community images
distributed as .tar.xz silently disappear from the redirect map and
return 404.

Add explicit cases for .tar.xz / .tar.gz / .tar.zst so the generator
emits the full compound extension.

Refs: #277
Refs: armbian/armbian-router#39

Author: SuperKali

scripts/generate-armbian-images-json.sh

@@ -467,6 +467,20 @@ extract_file_extension() {
     return
   fi
 
+  # tar archives (e.g. Arduino UNO Q images): *.tar.xz / *.tar.gz / *.tar.zst
+  if [[ "$n" == *.tar.xz ]]; then
+    echo "tar.xz"
+    return
+  fi
+  if [[ "$n" == *.tar.gz ]]; then
+    echo "tar.gz"
+    return
+  fi
+  if [[ "$n" == *.tar.zst ]]; then
+    echo "tar.zst"
+    return
+  fi
+
   # fallback
   echo "${n##*.}"
 }