diff --git a/qa/evil-tests/src/test/java/org/elasticsearch/index/store/LuceneFilesExtensionsTests.java b/qa/evil-tests/src/test/java/org/elasticsearch/index/store/LuceneFilesExtensionsTests.java
new file mode 100644
index 0000000000000..9248a2ca8e9fb
--- /dev/null
+++ b/qa/evil-tests/src/test/java/org/elasticsearch/index/store/LuceneFilesExtensionsTests.java
@@ -0,0 +1,43 @@
+/*
+ * Copyright Elasticsearch B.V. and/or licensed to Elasticsearch B.V. under one
+ * or more contributor license agreements. Licensed under the Elastic License
+ * 2.0 and the Server Side Public License, v 1; you may not use this file except
+ * in compliance with, at your election, the Elastic License 2.0 or the Server
+ * Side Public License, v 1.
+ */
+
+package org.elasticsearch.index.store;
+
+import org.elasticsearch.Assertions;
+import org.elasticsearch.core.SuppressForbidden;
+import org.elasticsearch.test.ESTestCase;
+
+import static org.hamcrest.Matchers.containsString;
+
+public class LuceneFilesExtensionsTests extends ESTestCase {
+
+    public void testUnknownFileExtension() {
+        if (Assertions.ENABLED) {
+            AssertionError e = expectThrows(AssertionError.class, () -> LuceneFilesExtensions.fromExtension("abc"));
+            assertThat(e.getMessage(), containsString("unknown Lucene file extension [abc]"));
+
+            setEsAllowUnknownLuceneFileExtensions("true");
+            try {
+                assertNull(LuceneFilesExtensions.fromExtension("abc"));
+            } finally {
+                setEsAllowUnknownLuceneFileExtensions(null);
+            }
+        } else {
+            assertNull(LuceneFilesExtensions.fromExtension("abc"));
+        }
+    }
+
+    @SuppressForbidden(reason = "set or clear system property es.allow_unknown_lucene_file_extensions")
+    public void setEsAllowUnknownLuceneFileExtensions(final String value) {
+        if (value == null) {
+            System.clearProperty("es.allow_unknown_lucene_file_extensions");
+        } else {
+            System.setProperty("es.allow_unknown_lucene_file_extensions", value);
+        }
+    }
+}
diff --git a/server/src/main/java/org/elasticsearch/index/store/LuceneFilesExtensions.java b/server/src/main/java/org/elasticsearch/index/store/LuceneFilesExtensions.java
index e173534795e6a..9d43fd4d7aa41 100644
--- a/server/src/main/java/org/elasticsearch/index/store/LuceneFilesExtensions.java
+++ b/server/src/main/java/org/elasticsearch/index/store/LuceneFilesExtensions.java
@@ -71,6 +71,16 @@ public enum LuceneFilesExtensions {
     // Lucene 9.0 indexed vectors metadata
     VEM("vem","Vector Metadata", true, false);
 
+    /**
+     * Allow plugin developers of custom codecs to opt out of the assertion in {@link #fromExtension}
+     * that checks that all encountered file extensions are known to this class.
+     * In the future, we would like to add a proper plugin extension point for this.
+     */
+    private static boolean allowUnknownLuceneFileExtensions() {
+        return Boolean.parseBoolean(
+            System.getProperty("es.allow_unknown_lucene_file_extensions", "false"));
+    }
+
     /**
      * Lucene file's extension.
      */
@@ -128,7 +138,7 @@ public boolean shouldMmap() {
     public static LuceneFilesExtensions fromExtension(String ext) {
         if (ext != null && ext.isEmpty() == false) {
             final LuceneFilesExtensions extension = extensions.get(ext);
-            assert extension != null: "unknown Lucene file extension [" + ext + ']';
+            assert allowUnknownLuceneFileExtensions() || extension != null: "unknown Lucene file extension [" + ext + ']';
             return extension;
         }
         return null;
diff --git a/x-pack/plugin/searchable-snapshots/src/main/java/org/elasticsearch/xpack/searchablesnapshots/cache/blob/BlobStoreCacheService.java b/x-pack/plugin/searchable-snapshots/src/main/java/org/elasticsearch/xpack/searchablesnapshots/cache/blob/BlobStoreCacheService.java
index 2f090d3ccf594..adeffe253efcf 100644
--- a/x-pack/plugin/searchable-snapshots/src/main/java/org/elasticsearch/xpack/searchablesnapshots/cache/blob/BlobStoreCacheService.java
+++ b/x-pack/plugin/searchable-snapshots/src/main/java/org/elasticsearch/xpack/searchablesnapshots/cache/blob/BlobStoreCacheService.java
@@ -260,7 +260,6 @@ public void onFailure(Exception e) {
      */
     public ByteRange computeBlobCacheByteRange(String fileName, long fileLength, ByteSizeValue maxMetadataLength) {
         final LuceneFilesExtensions fileExtension = LuceneFilesExtensions.fromExtension(IndexFileNames.getExtension(fileName));
-        assert fileExtension != null : "unknown Lucene file extension [" + fileName + "] - should it be considered a metadata file?";
 
         if (useLegacyCachedBlobSizes()) {
             if (fileLength <= ByteSizeUnit.KB.toBytes(8L)) {