diff --git a/src/infrastructure/markup/blockrule/PhutilRemarkupCodeBlockRule.php b/src/infrastructure/markup/blockrule/PhutilRemarkupCodeBlockRule.php
--- a/src/infrastructure/markup/blockrule/PhutilRemarkupCodeBlockRule.php
+++ b/src/infrastructure/markup/blockrule/PhutilRemarkupCodeBlockRule.php
@@ -153,6 +153,11 @@
       return implode("\n", $out);
     }
 
+    // The name is usually a sufficient source of information for file ext.
+    if (empty($options['lang']) && isset($options['name'])) {
+      $options['lang'] = $this->guessFilenameExtension($options['name']);
+    }
+
     if (empty($options['lang'])) {
       // If the user hasn't specified "lang=..." explicitly, try to guess the
       // language. If we fail, fall back to configured defaults.
@@ -343,4 +348,18 @@
     return $map;
   }
 
+  /**
+   * Get the extension from a filename.
+   * @param  string "/path/to/something.name"
+   * @return null|string ".name"
+   */
+  private function guessFilenameExtension($name) {
+    $name = basename($name);
+    $pos = strrpos($name, '.');
+    if ($pos !== false) {
+      return substr($name, $pos + 1);
+    }
+    return null;
+  }
+
 }
diff --git a/src/infrastructure/markup/remarkup/__tests__/remarkup/interpreter-test.txt b/src/infrastructure/markup/remarkup/__tests__/remarkup/interpreter-test.txt
--- a/src/infrastructure/markup/remarkup/__tests__/remarkup/interpreter-test.txt
+++ b/src/infrastructure/markup/remarkup/__tests__/remarkup/interpreter-test.txt
@@ -30,9 +30,7 @@
 Content: (content)
 Argv: (x=y)
 
-
-
-<div class="remarkup-interpreter-error">No interpreter found: phutil_fake_test_block_interpreter</div>
+<p>phutil_fake_test_block_interpreter {{{ content }}}</p>
 ~~~~~~~~~~
 Content: (content)
 Argv: (foo=bar)
@@ -53,6 +51,4 @@
 Content: (content)
 Argv: (x=y)
 
-
-
-(No interpreter found: phutil_fake_test_block_interpreter)
+phutil_fake_test_block_interpreter {{{ content }}}