Cast multipart threshold to integer (#11944)

config/initializers/paperclip.rb

@@ -25,7 +25,7 @@ if ENV['S3_ENABLED'] == 'true'
     s3_protocol: s3_protocol,
     s3_host_name: s3_hostname,
     s3_headers: {
-      'X-Amz-Multipart-Threshold' => ENV.fetch('S3_MULTIPART_THRESHOLD') { 15.megabytes },
+      'X-Amz-Multipart-Threshold' => ENV.fetch('S3_MULTIPART_THRESHOLD') { 15.megabytes }.to_i,
       'Cache-Control' => 'public, max-age=315576000, immutable',
     },
     s3_permissions: ENV.fetch('S3_PERMISSION') { 'public-read' },