The attachment suggest widget fails to upload files when using short URLs in Tomcat 9

Steps to reproduce

• Configure short URLs for Tomcat 9 using UrlRewriteFilter as described on the documentation, in particular with allowCasualMultipartParsing set to true.
• Make sure file uploads works as expected from the attachments tab.
• In a page, insert an attachment suggest widget with data-upload-allowed set to true (see example code below)
• Try to upload an attachment using the attachment suggest widget

{{velocity}}
{{html}}
#attachmentPicker({'data-upload-allowed': 'true'})
{{/html}}
{{/velocity}}

Actual result

The server returns an HTTP 500 with message "javax.mail.internet.ParseException: Missing start boundary".

Environment

• Recent Firefox or Chrome