[Bug]: Image editor silently writes PNG data into .JPG files (uppercase extension breaks canvas.toBlob MIME) #3424

Bug description

Editing a photo whose extension is uppercase .JPG in the built-in viewer image
editor corrupts the file: the stored bytes become PNG while the filename keeps its
.JPG extension.

Observed on Nextcloud 32.0.14.1, viewer 5.0.0, Chrome 155:

file on the server: PNG image data, 4320 x 3240, 8-bit/color RGBA.

The upload itself succeeds (PUT /remote.php/dav/files/<user>/<path>.JPG → 204),
so there is no indication anything went wrong.

Steps to reproduce

1. Upload a JPEG with an uppercase extension (e.g. ABC1234.JPG — the default
   export of many cameras).
2. Open it in the viewer (/apps/files/... → click the file), press Ctrl+E
   (or open with ?editing=true) so the image editor opens.
3. Apply any change (e.g. Filters → Black & White).
4. Click Save → Save image as dialog appears → Save → confirm overwrite.
5. Inspect the file server-side:

file data/<user>/files/ABC1234.JPG
# PNG image data, 4320 x 3240, 8-bit/color RGBA, non-interlaced

Expected behaviour

The file must still be a JPEG (or the user must be told the format changed).

Root cause

In src/components/ImageEditor.vue (viewer 5.0.0):

defaultSavedImageType() {
    return extname(this.decodedSourceName).slice(1) || 'jpeg'   // -> "JPG"
}

The extension is not lower-cased. The Filerobot editor then builds the MIME
type from it ('image/' + (ext === 'jpg' ? 'jpeg' : ext)) → image/JPG.

onSave() only normalises case-sensitively:

mimeType = mimeType.replace('jpg', 'jpeg')      // no-op for "image/JPG"
...
const blob = await new Promise(resolve => imageCanvas.toBlob(resolve, mimeType, quality))
const response = await axios.put(putUrl, new File([blob], fullName))

HTMLCanvasElement.toBlob() falls back to image/png for an unknown MIME type
(per spec: "if type is not a supported image format, use image/png"), so PNG
bytes are uploaded under the original .JPG name. The File's name is also not
checked against blob.type.

Consequences seen in the wild:

• PNG files that are 5–15× larger than the JPEG they replace (a 15 MB PNG was
  written over a 1.4 MB JPEG in a user report).
• Metadata/EXIF is lost, so Memories date/time editing then fails with
  Could not set exif data: Error: Not a valid JPG (looks more like a PNG) (HTTP 500).
• Previews, sync clients and third-party apps see a file whose extension lies
  about its content.

Suggested fix

defaultSavedImageType() {
    return (extname(this.decodedSourceName).slice(1) || 'jpeg').toLowerCase()
}

plus a defensive normalisation in onSave() (case-insensitive jpg → jpeg)
and, most importantly, a check after encoding:

if (blob.type !== mimeType) { showError(...); return }   // never upload mismatched bytes

Also worth lower-casing/validating the extension in the "Save image as" dialog
(format field), which currently keeps JPG as well.

Environment

• Nextcloud 32.0.14.1, PHP 8.3.33, viewer 5.0.0 (default install)
• Chrome 155, Linux
• Affected files: any image with an uppercase .JPG/.JPEG/.PNG-style extension;
  lowercase .jpg is unaffected (image/jpeg works).