src/Controller/TemporaryJsonapiFileFieldUploader.php | 11 ++++++++---
 1 file changed, 8 insertions(+), 3 deletions(-)

diff --git a/src/Controller/TemporaryJsonapiFileFieldUploader.php b/src/Controller/TemporaryJsonapiFileFieldUploader.php
index 6a97710..a33d8af 100644
--- a/src/Controller/TemporaryJsonapiFileFieldUploader.php
+++ b/src/Controller/TemporaryJsonapiFileFieldUploader.php
@@ -4,6 +4,7 @@ namespace Drupal\jsonapi\Controller;
 
 use Drupal\Component\Utility\Bytes;
 use Drupal\Component\Utility\Crypt;
+use Drupal\Component\Utility\Environment;
 use Drupal\Core\Entity\EntityInterface;
 use Drupal\Core\Entity\Plugin\DataType\EntityAdapter;
 use Drupal\Core\Field\FieldDefinitionInterface;
@@ -335,8 +336,8 @@ class TemporaryJsonapiFileFieldUploader {
       fclose($temp_file);
     }
     else {
-      // Close the file streams.
-      fclose($temp_file);
+      // Close the input file stream since we can't proceed with the upload.
+      // Don't try to close $temp_file since it's FALSE at this point.
       fclose($file_data);
       $this->logger->error('Temporary file "%path" could not be opened for file upload.', ['%path' => $temp_file_path]);
       throw new HttpException(500, 'Temporary file could not be opened');
@@ -463,7 +464,11 @@ class TemporaryJsonapiFileFieldUploader {
     $settings = $field_definition->getSettings();
 
     // Cap the upload size according to the PHP limit.
-    $max_filesize = Bytes::toInt(file_upload_max_size());
+    // @todo Remove when we stop supporting Drupal 8.5 and 8.6.
+    $max_bytes = floatval(\Drupal::VERSION) < 8.7
+      ? file_upload_max_size()
+      : Environment::getUploadMaxSize();
+    $max_filesize = Bytes::toInt($max_bytes);
     if (!empty($settings['max_filesize'])) {
       $max_filesize = min($max_filesize, Bytes::toInt($settings['max_filesize']));
     }