The preview hands the attachment's blob to the browser's own viewer,
which picks the viewer from the blob's type and downloads anything it
cannot show - under the blob's UUID. It only corrected the exact
application/octet-stream declaration from the file name. Other
declarations kept the preview from working: IBM i (AS/400) systems send
every attachment as application/x-as400attachment, some senders use
application/x-pdf or force-download, and the octet-stream check was
case-sensitive.
Move the decision into previewBlobType(): a PDF, image, audio or video
preview takes the first type its viewer can show - declared, then
served, then the one the file name implies. Other previews keep the
declared type unless it is generic, and the name never yields a
script-capable type.
Attachment thumbnails were object URLs typed with the sender's MIME
type. Object URLs share the webmail origin, so an `image/svg+xml`
attachment opened from its thumbnail ("Open image in new tab", or
dragged to the tab strip) ran the sender's script as the webmail in
Chromium and Firefox. The drag-out URL and Files thumbnails had the
same shape.
`fetchBlobAsObjectUrl` now re-types every blob to an inert type, and
image thumbnails in mail and Files go through `imageBlobUrl`, which
hands an SVG over as a `data:` URL: it still renders in the `<img>`,
but opened on its own it gets an opaque origin.
Linus Rath
dealerweb