Kiwi TCMS's misconfigured HTTP headers allow stored XSS execution with Firefox

Kiwi TCMS, an open source test management system allows users to upload attachments to test plans, test cases, etc. Versions of Kiwi TCMS prior to 12.5 had introduced changes which were meant to serve all uploaded files as plain text in order to prevent browsers from executing potentially dangerous files when such files are accessed directly. The previous Nginx configuration was incorrect allowing certain browsers like Firefox to ignore the `Content-Type: text/plain` header on some occasions thus allowing potentially dangerous scripts to be executed. Additionally, file upload validators and parts of the HTML rendering code had been found to require additional sanitation and improvements. Version 12.5 fixes this vulnerability with updated Nginx content type configuration, improved file upload validation code to prevent more potentially dangerous uploads, and Sanitization of test plan names used in the `tree_view_html()` function.