Large images are not handled consistently

When sending large images (more than 128 MegaPixel) there are 2 issues:

1. when sent as viewType Image ImageReader runs out of memory when trying to decode the image https://github.com/chatmail/core/blob/main/src/blob.rs#L360 which results in the fallback to viewType File

2. when the image is sent as viewType File core converts it to an image based on the mime type and "converts" it to viewType Image but does NOT run imgreader.decode() and does not rescale the image
https://github.com/chatmail/core/blob/main/src/blob.rs#L356

See https://github.com/deltachat/deltachat-desktop/issues/5053 and https://github.com/deltachat/deltachat-desktop/pull/6430

The desktop issue has an example image attached