tpl/reflect: Make the IsImageResource implementation less technical

And closer to the documentation. The old one was correct, but a bit too tied to the internal implementation.

resources/images/config.go

@@ -58,7 +58,8 @@ var (
 		".webp": WEBP,
 	}
 
-	imageFormatsBySubType = map[string]Format{
+	// These are the image types we can process.
+	processableImageSubTypes = map[string]Format{
 		media.Builtin.JPEGType.SubType: JPEG,
 		media.Builtin.PNGType.SubType:  PNG,
 		media.Builtin.TIFFType.SubType: TIFF,
@@ -123,7 +124,7 @@ func ImageFormatFromExt(ext string) (Format, bool) {
 }
 
 func ImageFormatFromMediaSubType(sub string) (Format, bool) {
-	f, found := imageFormatsBySubType[sub]
+	f, found := processableImageSubTypes[sub]
 	return f, found
 }
 

resources/transform.go

@@ -402,12 +402,16 @@ func (r *resourceAdapter) getImageOps() images.ImageResourceOps {
 
 // IsImage reports whether the given resource is an image that can be processed.
 func IsImage(v any) bool {
-	r, ok := v.(*resourceAdapter)
-	if ok {
-		_, ok := r.target.(images.ImageResourceOps)
-		return ok
+	r, ok := v.(resource.Resource)
+	if !ok {
+		return false
 	}
-	return false
+	mt := r.MediaType()
+	if mt.MainType != "image" {
+		return false
+	}
+	_, isImage := images.ImageFormatFromMediaSubType(mt.SubType)
+	return isImage
 }
 
 func (r *resourceAdapter) publish() {