diff --git a/lib/private/Files/ObjectStore/S3.php b/lib/private/Files/ObjectStore/S3.php
index 72e1751e23d..e92346e4ccc 100644
--- a/lib/private/Files/ObjectStore/S3.php
+++ b/lib/private/Files/ObjectStore/S3.php
@@ -100,6 +100,9 @@ class S3 implements IObjectStore, IObjectStoreMultiPartUpload, IObjectStoreMetaD
 		$result = [];
 		foreach ($metadata as $key => $value) {
 			if (str_starts_with($key, 'x-amz-meta-')) {
+				if (str_starts_with($value, 'base64:')) {
+					$value = base64_decode(substr($value, 7));
+				}
 				$result[substr($key, strlen('x-amz-meta-'))] = $value;
 			}
 		}
diff --git a/lib/private/Files/ObjectStore/S3ObjectTrait.php b/lib/private/Files/ObjectStore/S3ObjectTrait.php
index 9362609fa5a..f47a01dab18 100644
--- a/lib/private/Files/ObjectStore/S3ObjectTrait.php
+++ b/lib/private/Files/ObjectStore/S3ObjectTrait.php
@@ -82,7 +82,11 @@ trait S3ObjectTrait {
 	private function buildS3Metadata(array $metadata): array {
 		$result = [];
 		foreach ($metadata as $key => $value) {
-			$result['x-amz-meta-' . $key] = $value;
+			if (mb_check_encoding($value, 'ASCII')) {
+				$result['x-amz-meta-' . $key] = $value;
+			} else {
+				$result['x-amz-meta-' . $key] = 'base64:' . base64_encode($value);
+			}
 		}
 		return $result;
 	}