The isolated capsule receives a local File and has no network permission. Canvas decodes, redraws and serializes inside the tab.
No upload path
The capsule CSP uses connect-src none and remote image input is absent.
Local does not mean forensic
Canvas re-encoding is not a metadata sanitization, evidentiary or professional colour guarantee.
Verify the generated file
After resizing, compare the reported output dimensions and format with the requirements of the receiving website or document. Open the downloaded file there, inspect important edges and transparent areas, and keep the original until the result has been accepted. Check the preview at its natural size as well as inside the destination layout. If text, fine lines or a logo matter, zoom in and compare them with the source. A smaller byte count is useful evidence, but it does not by itself prove that the visual result is suitable.