Back then in 31e31ef, we added the Content-Disposition headers so that SVGs get downloaded instead of get run in the browser. Inadvertently, this also causes other attachments like pdfs and videos to be downloaded instead of heeding the "Open in new tab" option that users choose in the browser.
When the header is removed, the default value is "inline", this allows the browser to perform as requested. This also applies to other file types like pdfs, allowing users to "Open in new tab" and view them in the browser instead of always downloading them.
Existing tests (#10205) already do check that SVGs remain downloaded. Some existing tests written for PDFs have been modified to cater for SVGs instead, when there was a bug in defining the filenames per #10108
