diff --git a/src/wp-includes/functions.php b/src/wp-includes/functions.php
index ac4781d1f817b..a1b8dc934f791 100644
--- a/src/wp-includes/functions.php
+++ b/src/wp-includes/functions.php
@@ -3639,24 +3639,29 @@ function wp_get_ext_types() {
  * Wrapper for PHP filesize with filters and casting the result as an integer.
  *
  * @since 6.0.0
+ * @since 7.1.0 The return value is now always zero or greater.
  *
  * @link https://www.php.net/manual/en/function.filesize.php
  *
  * @param string $path Path to the file.
  * @return int The size of the file in bytes, or 0 in the event of an error.
+ * @phpstan-return non-negative-int
  */
-function wp_filesize( $path ) {
+function wp_filesize( $path ): int {
 	/**
 	 * Filters the result of wp_filesize() before the file_exists() PHP function is run.
 	 *
 	 * @since 6.0.0
+	 * @since 7.1.0 Negative values are now ignored, being treated the same as null. Numeric values are cast to integers.
 	 *
-	 * @param null|int $size The unfiltered value. Returning an int from the callback bypasses the filesize call.
+	 * @param null|int $size The unfiltered value. Returning a non-negative number from the callback bypasses the filesize call.
 	 * @param string   $path Path to the file.
 	 */
 	$size = apply_filters( 'pre_wp_filesize', null, $path );
-
-	if ( is_int( $size ) ) {
+	if ( is_numeric( $size ) ) {
+		$size = (int) $size;
+	}
+	if ( is_int( $size ) && $size >= 0 ) {
 		return $size;
 	}
 
@@ -3666,11 +3671,18 @@ function wp_filesize( $path ) {
 	 * Filters the size of the file.
 	 *
 	 * @since 6.0.0
+	 * @since 7.1.0 The return value is now always zero or greater. Numeric values are cast to integers.
 	 *
 	 * @param int    $size The result of PHP filesize on the file.
 	 * @param string $path Path to the file.
 	 */
-	return (int) apply_filters( 'wp_filesize', $size, $path );
+	$size = apply_filters( 'wp_filesize', $size, $path );
+	if ( is_numeric( $size ) ) {
+		$size = (int) $size;
+	} else {
+		$size = 0;
+	}
+	return max( 0, $size );
 }
 
 /**
diff --git a/src/wp-includes/rest-api/endpoints/class-wp-rest-attachments-controller.php b/src/wp-includes/rest-api/endpoints/class-wp-rest-attachments-controller.php
index 4737ee747a52b..c7a1cfc88bd2c 100644
--- a/src/wp-includes/rest-api/endpoints/class-wp-rest-attachments-controller.php
+++ b/src/wp-includes/rest-api/endpoints/class-wp-rest-attachments-controller.php
@@ -1707,7 +1707,7 @@ public function get_item_schema() {
 
 		$schema['properties']['filesize'] = array(
 			'description' => __( 'Attachment file size in bytes.' ),
-			'type'        => 'integer',
+			'type'        => array( 'integer', 'null' ),
 			'context'     => array( 'view', 'edit' ),
 			'readonly'    => true,
 		);
@@ -2359,12 +2359,13 @@ protected function get_attachment_filename( int $attachment_id ): ?string {
 	 *
 	 * @param int $attachment_id Attachment ID.
 	 * @return int|null Attachment file size in bytes, or null if not available.
+	 * @phpstan-return non-negative-int|null
 	 */
 	protected function get_attachment_filesize( int $attachment_id ): ?int {
 		$meta = wp_get_attachment_metadata( $attachment_id );
 
-		if ( isset( $meta['filesize'] ) ) {
-			return $meta['filesize'];
+		if ( isset( $meta['filesize'] ) && is_numeric( $meta['filesize'] ) && $meta['filesize'] > 0 ) {
+			return (int) $meta['filesize'];
 		}
 
 		$original_path = wp_get_original_image_path( $attachment_id );
diff --git a/tests/phpunit/tests/functions/wpFilesize.php b/tests/phpunit/tests/functions/wpFilesize.php
index 4ed6de5676bd2..07a62ac2004aa 100644
--- a/tests/phpunit/tests/functions/wpFilesize.php
+++ b/tests/phpunit/tests/functions/wpFilesize.php
@@ -9,46 +9,100 @@
  */
 class Tests_Functions_wpFilesize extends WP_UnitTestCase {
 
+	const TEST_FILE = DIR_TESTDATA . '/images/test-image-upside-down.jpg';
+
 	/**
 	 * @ticket 49412
 	 */
-	public function test_wp_filesize() {
-		$file = DIR_TESTDATA . '/images/test-image-upside-down.jpg';
-
-		$this->assertSame( filesize( $file ), wp_filesize( $file ) );
+	public function test_wp_filesize(): void {
+		$this->assertSame( filesize( self::TEST_FILE ), wp_filesize( self::TEST_FILE ) );
 	}
 
 	/**
 	 * @ticket 49412
+	 * @ticket 65670
 	 */
-	public function test_wp_filesize_filters() {
-		$file = DIR_TESTDATA . '/images/test-image-upside-down.jpg';
+	public function test_wp_filesize_filters(): void {
+		add_filter( 'wp_filesize', static fn () => 999 );
+		$this->assertSame( 999, wp_filesize( self::TEST_FILE ) );
 
-		add_filter(
-			'wp_filesize',
-			static function () {
-				return 999;
-			}
-		);
+		add_filter( 'wp_filesize', static fn () => '9991', 100 );
+		$this->assertSame( 9991, wp_filesize( self::TEST_FILE ) );
 
-		$this->assertSame( 999, wp_filesize( $file ) );
+		add_filter( 'pre_wp_filesize', static fn () => 111 );
+		$this->assertSame( 111, wp_filesize( self::TEST_FILE ) );
 
-		add_filter(
-			'pre_wp_filesize',
-			static function () {
-				return 111;
-			}
-		);
+		add_filter( 'pre_wp_filesize', static fn () => '2222', 100 );
+		$this->assertSame( 2222, wp_filesize( self::TEST_FILE ) );
 
-		$this->assertSame( 111, wp_filesize( $file ) );
+		add_filter( 'pre_wp_filesize', static fn () => -100, 200 );
+		$this->assertSame( 9991, wp_filesize( self::TEST_FILE ) );
 	}
 
 	/**
 	 * @ticket 49412
 	 */
-	public function test_wp_filesize_with_nonexistent_file() {
-		$file = 'nonexistent/file.jpg';
+	public function test_wp_filesize_with_nonexistent_file(): void {
+		$this->assertSame( 0, wp_filesize( 'nonexistent/file.jpg' ) );
+	}
+
+	/**
+	 * @ticket 65670
+	 */
+	public function test_wp_filesize_pre_wp_filesize_filter_null(): void {
+		add_filter( 'pre_wp_filesize', '__return_null' );
+
+		$this->assertSame( filesize( self::TEST_FILE ), wp_filesize( self::TEST_FILE ) );
+	}
 
-		$this->assertSame( 0, wp_filesize( $file ) );
+	/**
+	 * @ticket 65670
+	 *
+	 * @dataProvider data_wp_filesize_pre_wp_filesize_filter_negative
+	 *
+	 * @param float|int|string $value Negative value returned by the filter.
+	 */
+	public function test_wp_filesize_pre_wp_filesize_filter_negative( $value ): void {
+		add_filter( 'pre_wp_filesize', static fn () => $value );
+
+		$this->assertSame( filesize( self::TEST_FILE ), wp_filesize( self::TEST_FILE ) );
+	}
+
+	/**
+	 * Data provider.
+	 *
+	 * @return array<string, array{ 0: float|int|string }>
+	 */
+	public function data_wp_filesize_pre_wp_filesize_filter_negative(): array {
+		return array(
+			'negative int'            => array( -1 ),
+			'negative numeric string' => array( '-1' ),
+			'negative float'          => array( -1.5 ),
+		);
+	}
+
+	/**
+	 * @ticket 65670
+	 *
+	 * @dataProvider data_wp_filesize_filter_invalid_value
+	 *
+	 * @param mixed $value
+	 */
+	public function test_wp_filesize_wp_filesize_filter_invalid_value( $value ): void {
+		add_filter( 'wp_filesize', static fn () => $value );
+		$this->assertSame( 0, wp_filesize( self::TEST_FILE ) );
+	}
+
+	/**
+	 * Data provider.
+	 *
+	 * @return array<string, array{ 0: mixed }>
+	 */
+	public function data_wp_filesize_filter_invalid_value(): array {
+		return array(
+			'negative' => array( -1 ),
+			'null'     => array( null ),
+			'array'    => array( array( 'bad', 'array' ) ),
+		);
 	}
 }
diff --git a/tests/phpunit/tests/rest-api/rest-attachments-controller.php b/tests/phpunit/tests/rest-api/rest-attachments-controller.php
index 911676614754d..2b277c11d96a7 100644
--- a/tests/phpunit/tests/rest-api/rest-attachments-controller.php
+++ b/tests/phpunit/tests/rest-api/rest-attachments-controller.php
@@ -1005,6 +1005,111 @@ public function test_get_item() {
 		$this->assertSame( 'image/jpeg', $data['mime_type'] );
 	}
 
+	/**
+	 * Ensures int-castable `filesize` values in attachment metadata are normalized
+	 * to an integer in the response.
+	 *
+	 * Attachment metadata is untyped, so plugins that populate `filesize` from
+	 * a remote storage API may store it as a string.
+	 *
+	 * @ticket 65670
+	 *
+	 * @dataProvider data_valid_filesize_meta
+	 *
+	 * @param mixed $stored_filesize   Valid `filesize` metadata value.
+	 * @param int   $expected_filesize Expected `filesize` value in the REST response after normalization.
+	 */
+	public function test_get_item_normalizes_int_castable_filesize_meta( $stored_filesize, int $expected_filesize ) {
+		$attachment_id = self::factory()->attachment->create_object(
+			array(
+				'file'           => self::$test_file,
+				'post_mime_type' => 'image/jpeg',
+			)
+		);
+		$this->assertIsInt( $attachment_id );
+
+		$meta             = wp_get_attachment_metadata( $attachment_id );
+		$meta             = is_array( $meta ) ? $meta : array();
+		$meta['filesize'] = $stored_filesize;
+		$this->assertNotFalse( wp_update_attachment_metadata( $attachment_id, $meta ) );
+
+		$request  = new WP_REST_Request( 'GET', '/wp/v2/media/' . $attachment_id );
+		$response = rest_get_server()->dispatch( $request );
+		$data     = $response->get_data();
+
+		$this->assertIsArray( $data );
+		$this->assertSame( 200, $response->get_status() );
+		$this->assertSame( $expected_filesize, $data['filesize'] );
+	}
+
+	/**
+	 * Data provider.
+	 *
+	 * @return array<string, array{ 0: mixed, 1: int }>
+	 */
+	public function data_valid_filesize_meta(): array {
+		return array(
+			'integer string'      => array( '123456', 123456 ),
+			'float string'        => array( '123.4', 123 ),
+			'scientific notation' => array( '1e3', 1000 ),
+			'float'               => array( 123.0, 123 ),
+		);
+	}
+
+	/**
+	 * Ensures a `filesize` metadata value that is not a positive integer does
+	 * not cause a fatal TypeError or a silently truncated size, falling back to
+	 * the actual file size instead.
+	 *
+	 * @ticket 65670
+	 *
+	 * @dataProvider data_invalid_filesize_meta
+	 *
+	 * @param mixed $filesize Invalid `filesize` metadata value.
+	 */
+	public function test_get_item_recovers_from_invalid_filesize_meta( $filesize ) {
+		$attachment_id = self::factory()->attachment->create_object(
+			array(
+				'file'           => self::$test_file,
+				'post_mime_type' => 'image/jpeg',
+			)
+		);
+		$this->assertIsInt( $attachment_id );
+
+		$meta             = wp_get_attachment_metadata( $attachment_id );
+		$meta             = is_array( $meta ) ? $meta : array();
+		$meta['filesize'] = $filesize;
+		$this->assertNotFalse( wp_update_attachment_metadata( $attachment_id, $meta ) );
+
+		$request  = new WP_REST_Request( 'GET', '/wp/v2/media/' . $attachment_id );
+		$response = rest_get_server()->dispatch( $request );
+		$data     = $response->get_data();
+
+		$this->assertIsArray( $data );
+		$this->assertSame( 200, $response->get_status() );
+		$this->assertIsInt( $data['filesize'] );
+		$attached_file = wp_get_original_image_path( $attachment_id );
+		$attached_file = $attached_file ? $attached_file : get_attached_file( $attachment_id );
+		$this->assertIsString( $attached_file );
+		$this->assertSame( filesize( $attached_file ), $data['filesize'] );
+	}
+
+	/**
+	 * Data provider.
+	 *
+	 * @return array<string, array{ 0: mixed }>
+	 */
+	public function data_invalid_filesize_meta(): array {
+		return array(
+			'non-numeric string'      => array( 'corrupt' ),
+			'boolean'                 => array( true ),
+			'zero'                    => array( 0 ),
+			'zero string'             => array( '0' ),
+			'negative integer'        => array( -5 ),
+			'negative integer string' => array( '-5' ),
+		);
+	}
+
 	/**
 	 * @requires function imagejpeg
 	 */