diff --git a/extension.json b/extension.json
index 421ca57..bccf6db 100644
--- a/extension.json
+++ b/extension.json
@@ -7,7 +7,7 @@
 	"license-name": "MIT",
 	"@note": "Only list specific tested MediaWiki versions given how unstable the extension is",
 	"requires": {
-		"MediaWiki": "1.39.15 || 1.43.5"
+		"MediaWiki": "1.39.17 || 1.43.5"
 	},
 	"AutoloadNamespaces": {
 		"MediaWiki\\Extension\\MixedVisibilityFiles\\": "src/"
@@ -27,7 +27,9 @@
 		"visibility": {
 			"class": "\\MediaWiki\\Extension\\MixedVisibilityFiles\\VisibilityHooks",
 			"services": [
-				"PageProps"
+				"PageProps",
+				"UserGroupManager",
+				"MainConfig"
 			]
 		}
 	},
@@ -36,5 +38,11 @@
 		"getUserPermissionsErrorsExpensive": "visibility",
 		"MediaWikiServices": "services"
 	},
+	"config": {
+		"MixedVisibilityFilesPrefixRestrictions": {
+			"value": {},
+			"description": "Map of file name prefixes to user group(s). Files whose name starts with a given prefix are restricted to users in the specified group(s). Each key is a prefix string and each value is either a group name (string) or an array of group names. Prefix matching is case-sensitive; note that MediaWiki normalizes the first character of file names to uppercase. Example: {\"Confidential_\": \"sysop\", \"Internal_\": [\"sysop\", \"bureaucrat\"]}"
+		}
+	},
 	"manifest_version": 2
 }
diff --git a/src/VisibilityHooks.php b/src/VisibilityHooks.php
index 1da3d7d..9ae05d1 100644
--- a/src/VisibilityHooks.php
+++ b/src/VisibilityHooks.php
@@ -2,8 +2,10 @@
 
 namespace MediaWiki\Extension\MixedVisibilityFiles;
 
+use Config;
 use MediaWiki\Hook\GetDoubleUnderscoreIDsHook;
 use MediaWiki\Permissions\Hook\GetUserPermissionsErrorsExpensiveHook;
+use MediaWiki\User\UserGroupManager;
 use PageProps;
 
 class VisibilityHooks implements
@@ -13,8 +15,19 @@ class VisibilityHooks implements
 
 	private PageProps $pageProps;
 
-	public function __construct( PageProps $pageProps ) {
+	private UserGroupManager $userGroupManager;
+
+	/** @var array<string,string|string[]> */
+	private array $prefixRestrictions;
+
+	public function __construct(
+		PageProps $pageProps,
+		UserGroupManager $userGroupManager,
+		Config $config
+	) {
 		$this->pageProps = $pageProps;
+		$this->userGroupManager = $userGroupManager;
+		$this->prefixRestrictions = $config->get( 'MixedVisibilityFilesPrefixRestrictions' );
 	}
 
 	/**
@@ -33,14 +46,36 @@ public function onGetUserPermissionsErrorsExpensive( $title, $user, $action,
 		if ( MW_ENTRY_POINT !== 'img_auth' ) {
 			return;
 		}
-		// Only trying to affect file reads by anonymous users
+		// Only trying to affect file reads
 		if ( $action !== 'read' ) {
 			return;
 		}
-		if ( $user->isRegistered() ) {
+		if ( $title->getNamespace() !== NS_FILE ) {
 			return;
 		}
-		if ( $title->getNamespace() !== NS_FILE ) {
+
+		// Check prefix-based group restrictions (applies to all users)
+		$fileName = $title->getDBkey();
+		foreach ( $this->prefixRestrictions as $prefix => $requiredGroups ) {
+			if ( str_starts_with( $fileName, $prefix ) ) {
+				// File matches a prefix restriction
+				if ( !$user->isRegistered() ) {
+					$result = false;
+					return false;
+				}
+				$userGroups = $this->userGroupManager->getUserEffectiveGroups( $user );
+				$allowedGroups = (array)$requiredGroups;
+				if ( !array_intersect( $allowedGroups, $userGroups ) ) {
+					$result = false;
+					return false;
+				}
+				// User is in a required group, allow access
+				return;
+			}
+		}
+
+		// Default behavior: block anonymous users unless file is marked public
+		if ( $user->isRegistered() ) {
 			return;
 		}