Tiled Gallery: handle failed uploads gracefully

### Impacted plugin

Jetpack

### Quick summary

If one tries to upload a disallowed image file type (e.g. `.avif`), it shows the image preview during "upload", and then the image disappears without any message.

Reported in p8oabR-1wt-p2#comment-8009.

### Steps to reproduce

See summary.

### A clear and concise description of what you expected to happen.

At minimum there should be some messaging to let the user know that not all the selected images were uploaded. In an ideal world, it'd list which file(s) failed.

### What actually happened

See summary.

### Impact

Some (< 50%)

### Available workarounds?

No but the platform is still usable

### Platform (Simple and/or Atomic)

_No response_

### Logs or notes

_No response_

Start by reproducing the failed disallowed-image upload in Jetpack's Tiled Gallery and trace the upload flow responsible for the preview disappearing. The issue names no files or tests; done means failed uploads show a clear message, ideally identifying each file that was rejected.

Written by the indexing model from the issue text.