diff --git a/lib/composer/composer/autoload_classmap.php b/lib/composer/composer/autoload_classmap.php
index be06cdd45b860..a84bd24af4dd8 100644
--- a/lib/composer/composer/autoload_classmap.php
+++ b/lib/composer/composer/autoload_classmap.php
@@ -1056,6 +1056,7 @@
     'OCP\\TaskProcessing\\TaskTypes\\TextToTextSummary' => $baseDir . '/lib/public/TaskProcessing/TaskTypes/TextToTextSummary.php',
     'OCP\\TaskProcessing\\TaskTypes\\TextToTextTopics' => $baseDir . '/lib/public/TaskProcessing/TaskTypes/TextToTextTopics.php',
     'OCP\\TaskProcessing\\TaskTypes\\TextToTextTranslate' => $baseDir . '/lib/public/TaskProcessing/TaskTypes/TextToTextTranslate.php',
+    'OCP\\Teams\\ITeamFileResolver' => $baseDir . '/lib/public/Teams/ITeamFileResolver.php',
     'OCP\\Teams\\ITeamFolderProvider' => $baseDir . '/lib/public/Teams/ITeamFolderProvider.php',
     'OCP\\Teams\\ITeamManager' => $baseDir . '/lib/public/Teams/ITeamManager.php',
     'OCP\\Teams\\ITeamResourceProvider' => $baseDir . '/lib/public/Teams/ITeamResourceProvider.php',
diff --git a/lib/composer/composer/autoload_static.php b/lib/composer/composer/autoload_static.php
index 73277b5502fe1..5994bcd56b3b6 100644
--- a/lib/composer/composer/autoload_static.php
+++ b/lib/composer/composer/autoload_static.php
@@ -1097,6 +1097,7 @@ class ComposerStaticInit749170dad3f5e7f9ca158f5a9f04f6a2
         'OCP\\TaskProcessing\\TaskTypes\\TextToTextSummary' => __DIR__ . '/../../..' . '/lib/public/TaskProcessing/TaskTypes/TextToTextSummary.php',
         'OCP\\TaskProcessing\\TaskTypes\\TextToTextTopics' => __DIR__ . '/../../..' . '/lib/public/TaskProcessing/TaskTypes/TextToTextTopics.php',
         'OCP\\TaskProcessing\\TaskTypes\\TextToTextTranslate' => __DIR__ . '/../../..' . '/lib/public/TaskProcessing/TaskTypes/TextToTextTranslate.php',
+        'OCP\\Teams\\ITeamFileResolver' => __DIR__ . '/../../..' . '/lib/public/Teams/ITeamFileResolver.php',
         'OCP\\Teams\\ITeamFolderProvider' => __DIR__ . '/../../..' . '/lib/public/Teams/ITeamFolderProvider.php',
         'OCP\\Teams\\ITeamManager' => __DIR__ . '/../../..' . '/lib/public/Teams/ITeamManager.php',
         'OCP\\Teams\\ITeamResourceProvider' => __DIR__ . '/../../..' . '/lib/public/Teams/ITeamResourceProvider.php',
diff --git a/lib/private/Teams/TeamManager.php b/lib/private/Teams/TeamManager.php
index d17aefe36223b..8289150b2e521 100644
--- a/lib/private/Teams/TeamManager.php
+++ b/lib/private/Teams/TeamManager.php
@@ -15,6 +15,7 @@
 use OCA\Circles\Model\Probes\CircleProbe;
 use OCP\IURLGenerator;
 use OCP\Server;
+use OCP\Teams\ITeamFileResolver;
 use OCP\Teams\ITeamFolderProvider;
 use OCP\Teams\ITeamManager;
 use OCP\Teams\ITeamResourceProvider;
@@ -23,6 +24,11 @@
 use Psr\Container\NotFoundExceptionInterface;
 
 class TeamManager implements ITeamManager {
+	/**
+	 * Id of the provider whose resource ids are file ids. Owned by the circles
+	 * app, and hardcoded by every caller of the teams resource route.
+	 */
+	private const FILE_ADDRESSED_PROVIDER_ID = 'files';
 
 	/** @var ?ITeamResourceProvider[] */
 	private ?array $providers = null;
@@ -130,8 +136,31 @@ public function getTeamsForResource(string $providerId, string $resourceId, stri
 			return [];
 		}
 
+		return array_map($this->circleToTeam(...), $this->getTeams($this->collectTeamIds($providerId, $resourceId), $userId));
+	}
+
+	/**
+	 * When the lookup addressed resources by file id, ask every file resolver as
+	 * well: a team folder is mounted rather than shared, so a share lookup alone
+	 * never finds it.
+	 *
+	 * @return list<string>
+	 */
+	private function collectTeamIds(string $providerId, string $resourceId): array {
 		$provider = $this->getProvider($providerId);
-		return array_map($this->circleToTeam(...), $this->getTeams($provider->getTeamsForResource($resourceId), $userId));
+		$teamIds = $provider->getTeamsForResource($resourceId);
+
+		if ($providerId === self::FILE_ADDRESSED_PROVIDER_ID) {
+			foreach ($this->getProviders() as $candidate) {
+				if ($candidate === $provider || !$candidate instanceof ITeamFileResolver) {
+					continue;
+				}
+
+				$teamIds = array_merge($teamIds, $candidate->getTeamsForFile((int)$resourceId));
+			}
+		}
+
+		return array_values(array_unique($teamIds));
 	}
 
 	private function getTeamInternal(string $teamId, string $userId, ?CircleProbe $probe = null): ?Circle {
diff --git a/lib/public/Teams/ITeamFileResolver.php b/lib/public/Teams/ITeamFileResolver.php
new file mode 100644
index 0000000000000..c7eadf18211f6
--- /dev/null
+++ b/lib/public/Teams/ITeamFileResolver.php
@@ -0,0 +1,39 @@
+<?php
+
+declare(strict_types=1);
+
+/**
+ * SPDX-FileCopyrightText: 2026 Nextcloud GmbH and Nextcloud contributors
+ * SPDX-License-Identifier: AGPL-3.0-or-later
+ */
+
+namespace OCP\Teams;
+
+use OCP\AppFramework\Attribute\Consumable;
+use OCP\AppFramework\Attribute\Implementable;
+
+/**
+ * Resolves the teams related to a file, whatever this provider's own resource
+ * ids happen to be.
+ *
+ * Asked in addition to the provider that a lookup addressed, whenever that
+ * lookup addressed resources by file id. That lets a relation one provider
+ * cannot see be contributed by another: a team folder is mounted rather than
+ * shared, so a share lookup will never find it.
+ *
+ * @since 35.0.0
+ */
+#[Consumable(since: '35.0.0')]
+#[Implementable(since: '35.0.0')]
+interface ITeamFileResolver extends ITeamResourceProvider {
+	/**
+	 * Return the ids of the teams the given file belongs to.
+	 *
+	 * May include teams the current user cannot see; the caller filters those out.
+	 *
+	 * @param int $fileId Id of the file as stored in the file cache.
+	 * @return list<string> Team ids, empty when the file is unrelated to this provider.
+	 * @since 35.0.0
+	 */
+	public function getTeamsForFile(int $fileId): array;
+}
diff --git a/tests/lib/Teams/TeamManagerTest.php b/tests/lib/Teams/TeamManagerTest.php
index b5b1a42aca124..37661664ee537 100644
--- a/tests/lib/Teams/TeamManagerTest.php
+++ b/tests/lib/Teams/TeamManagerTest.php
@@ -12,6 +12,7 @@
 use OC\AppFramework\Bootstrap\Coordinator;
 use OC\Teams\TeamManager;
 use OCP\IURLGenerator;
+use OCP\Teams\ITeamFileResolver;
 use OCP\Teams\ITeamFolderProvider;
 use OCP\Teams\ITeamResourceProvider;
 use Test\TestCase;
@@ -43,6 +44,75 @@ public function testGetTeamFolderProviderReturnsRegisteredFolderProvider(): void
 		$this->assertSame($folderProvider, $teamManager->getTeamFolderProvider());
 	}
 
+	public function testCollectTeamIdsDoesNotWidenForOtherProviders(): void {
+		$teamManager = $this->createTeamManager(true);
+		$addressed = $this->createMock(ITeamResourceProvider::class);
+		$addressed->expects($this->once())
+			->method('getTeamsForResource')
+			->with('42')
+			->willReturn(['team-1']);
+
+		$resolver = $this->createMock(ITeamFileResolver::class);
+		$resolver->expects($this->never())->method('getTeamsForFile');
+
+		$this->setProviders($teamManager, ['deck' => $addressed, 'groupfolders' => $resolver]);
+
+		$this->assertSame(['team-1'], $this->collectTeamIds($teamManager, 'deck', '42'));
+	}
+
+	public function testCollectTeamIdsAsksEveryResolverForTheFilesProvider(): void {
+		$teamManager = $this->createTeamManager(true);
+		$addressed = $this->createMock(ITeamResourceProvider::class);
+		$addressed->method('getTeamsForResource')->with('42')->willReturn(['team-1']);
+
+		$resolver = $this->createMock(ITeamFileResolver::class);
+		$resolver->expects($this->once())
+			->method('getTeamsForFile')
+			->with(42)
+			->willReturn(['team-2']);
+
+		$unrelated = $this->createMock(ITeamResourceProvider::class);
+		$unrelated->expects($this->never())->method('getTeamsForResource');
+
+		$this->setProviders($teamManager, [
+			'files' => $addressed,
+			'groupfolders' => $resolver,
+			'talk' => $unrelated,
+		]);
+
+		$this->assertSame(['team-1', 'team-2'], $this->collectTeamIds($teamManager, 'files', '42'));
+	}
+
+	public function testCollectTeamIdsDoesNotAskTheAddressedProviderTwice(): void {
+		$teamManager = $this->createTeamManager(true);
+		$addressed = $this->createMock(ITeamFileResolver::class);
+		$addressed->method('getTeamsForResource')->willReturn(['team-1']);
+		$addressed->expects($this->never())->method('getTeamsForFile');
+
+		$this->setProviders($teamManager, ['files' => $addressed]);
+
+		$this->assertSame(['team-1'], $this->collectTeamIds($teamManager, 'files', '42'));
+	}
+
+	public function testCollectTeamIdsReturnsEachTeamOnce(): void {
+		$teamManager = $this->createTeamManager(true);
+		$addressed = $this->createMock(ITeamResourceProvider::class);
+		$addressed->method('getTeamsForResource')->willReturn(['team-1']);
+
+		$resolver = $this->createMock(ITeamFileResolver::class);
+		$resolver->method('getTeamsForFile')->willReturn(['team-1', 'team-2']);
+
+		$this->setProviders($teamManager, ['files' => $addressed, 'groupfolders' => $resolver]);
+
+		$this->assertSame(['team-1', 'team-2'], $this->collectTeamIds($teamManager, 'files', '42'));
+	}
+
+	private function collectTeamIds(TeamManager $teamManager, string $providerId, string $resourceId): array {
+		$method = new \ReflectionMethod(TeamManager::class, 'collectTeamIds');
+
+		return $method->invoke($teamManager, $providerId, $resourceId);
+	}
+
 	private function createTeamManager(bool $hasTeamSupport = false): TeamManager {
 		return new class($this->createMock(Coordinator::class), $this->createMock(IURLGenerator::class), null, $hasTeamSupport, ) extends TeamManager {
 			public function __construct(