Automatic thumbnails do not update when source image is replaced #2066

Expected behaviour

An automatic thumbnail resembles the appearance of the source image throughout its entire life cycle.

Actual behaviour

The automatic thumbnail resembles the first uploaded image file, even if the source image is replaced afterwards.

Steps to reproduce

1. Open the ?event=image tab and upload an image file (say image-1).
2. Open the ?event=image&step=image_edit tab with this newly uploaded image. Verify its proper look. Let the "Thumbnail type" be "Automatic". The generated thumbnail meets your expectations: A scaled-down version of image-1.
3. Hit the "Replace image" button and upload a different image (say image-2).
4. The thumbnail image stays unmodified and still shows small version of image-1.

I cannot find a user-friendly way of creating a thumbnail that replicates image-2 without deleting the thumbnail file on the server's file system (e.g. /images/thumb/w200-q80/42.png) . This makes me think that the mere existence of the thumbnail file for a particular image id restricts any updates to the thumbnail content, regardless of whether the source image remains the same as the one used to create the thumbnail.

Additional information

Textpattern version: 4.9.2-dev (ebc2de647de9d9f73620b91047fca39ddab84655f47ad4da)