diff --git a/inc/class-plugin.php b/inc/class-plugin.php
index 50c2b015..94d73706 100644
--- a/inc/class-plugin.php
+++ b/inc/class-plugin.php
@@ -552,22 +552,22 @@ public function set_attachment_files_acl( int $attachment_id, string $acl ) : ?W
 	 * @return list<string> Array of all full paths to the attachment's files.
 	 */
 	public static function get_attachment_files( int $attachment_id ) : array {
-		$uploadpath = wp_get_upload_dir();
 		/** @var string */
 		$main_file = get_attached_file( $attachment_id );
+		$main_file_directory = dirname( $main_file );
 		$files = [ $main_file ];
 
 		$meta = wp_get_attachment_metadata( $attachment_id );
 		if ( isset( $meta['sizes'] ) ) {
 			foreach ( $meta['sizes'] as $size => $sizeinfo ) {
-				$files[] = $uploadpath['basedir'] . $sizeinfo['file'];
+				$files[] = $main_file_directory . '/' . $sizeinfo['file'];
 			}
 		}
 
 		/** @var string|false */
 		$original_image = get_post_meta( $attachment_id, 'original_image', true );
 		if ( $original_image ) {
-			$files[] = $uploadpath['basedir'] . $original_image;
+			$files[] = $main_file_directory . '/' . $original_image;
 		}
 
 		/** @var array<string,array{file: string}> */
@@ -576,7 +576,7 @@ public static function get_attachment_files( int $attachment_id ) : array {
 			foreach ( $backup_sizes as $size => $sizeinfo ) {
 				// Backup sizes only store the backup filename, which is relative to the
 				// main attached file, unlike the metadata sizes array.
-				$files[] = path_join( dirname( $main_file ), $sizeinfo['file'] );
+				$files[] = $main_file_directory . '/' . $sizeinfo['file'];
 			}
 		}
 
diff --git a/tests/test-s3-uploads.php b/tests/test-s3-uploads.php
index a0b7fd04..22b98b17 100644
--- a/tests/test-s3-uploads.php
+++ b/tests/test-s3-uploads.php
@@ -183,4 +183,61 @@ function test_wp_unique_filename() {
 		$this->assertEquals( 'my-new-file.jpg', $filename );
 	}
 
+	function test_get_attachment_files() {
+		S3_Uploads\Plugin::get_instance()->setup();
+		$upload_dir = wp_upload_dir();
+		copy( dirname( __FILE__ ) . '/data/sunflower.jpg', $upload_dir['path'] . '/sunflower.jpg' );
+		$test_file = $upload_dir['path'] . '/sunflower.jpg';
+		$attachment_id = self::factory()->attachment->create_object( $test_file, 0, array(
+			'post_mime_type' => 'image/jpeg',
+			'post_excerpt'   => 'A sample caption',
+		) );
+
+		$files = S3_Uploads\Plugin::get_instance()->get_attachment_files( $attachment_id );
+		$this->assertIsArray( $files );
+		$this->assertNotEmpty( $files );
+		$this->assertContains( $upload_dir['path'] . '/sunflower.jpg', $files );
+	}
+
+	function test_get_attachment_files_intermediate_sizes() {
+		S3_Uploads\Plugin::get_instance()->setup();
+		$upload_dir = wp_upload_dir();
+		copy( dirname( __FILE__ ) . '/data/sunflower.jpg', $upload_dir['path'] . '/sunflower.jpg' );
+		$test_file = $upload_dir['path'] . '/sunflower.jpg';
+		$attachment_id = self::factory()->attachment->create_object( $test_file, 0, array(
+			'post_mime_type' => 'image/jpeg',
+			'post_excerpt'   => 'A sample caption',
+		) );
+
+		// Generate metadata to create intermediate sizes
+		wp_generate_attachment_metadata( $attachment_id, $test_file );
+
+		$files = S3_Uploads\Plugin::get_instance()->get_attachment_files( $attachment_id );
+		$this->assertIsArray( $files );
+		$this->assertNotEmpty( $files );
+		$this->assertContains( $upload_dir['path'] . '/sunflower-300x196.jpg', $files );
+		$this->assertContains( $upload_dir['path'] . '/sunflower-150x150.jpg', $files );
+	}
+
+	function test_get_attachment_files_intermediate_sizes_in_subdir() {
+		S3_Uploads\Plugin::get_instance()->setup();
+		$upload_dir = wp_upload_dir();
+		copy( dirname( __FILE__ ) . '/data/sunflower.jpg', $upload_dir['path'] . '/a/sub/dir/sunflower.jpg' );
+		$test_file = $upload_dir['path'] . '/a/sub/dir/sunflower.jpg';
+		$attachment_id = self::factory()->attachment->create_object( $test_file, 0, array(
+			'post_mime_type' => 'image/jpeg',
+			'post_excerpt'   => 'A sample caption',
+		) );
+
+		// Generate metadata to create intermediate sizes
+		wp_generate_attachment_metadata( $attachment_id, $test_file );
+
+		$files = S3_Uploads\Plugin::get_instance()->get_attachment_files( $attachment_id );
+		$this->assertIsArray( $files );
+		$this->assertNotEmpty( $files );
+		$this->assertContains( $upload_dir['path'] . '/a/sub/dir/sunflower.jpg', $files );
+		$this->assertContains( $upload_dir['path'] . '/a/sub/dir/sunflower-300x196.jpg', $files );
+		$this->assertContains( $upload_dir['path'] . '/a/sub/dir/sunflower-150x150.jpg', $files );
+	}
+
 }