Since 3765ac1f1f16, uploading an image not recognized as such by PIL would not
behaves as it should.
Step to reproduce:
- Enter edit mode, drag & drop a snippet with an image
- Double click on the image to open media dialog
- Upload a local image of type `.webp`
- 2 toaster are shown:
1. A progress bar toaster with a generic message "File could not be saved"
2. A generic warning toaster on top of the previous one with the correct
error message "[..] format is not supported. Try with: .gif [..]"
Instead, only the progress bar toaster should be shown, and the error message
should be displayed in it.
task-2607393
Closes#74122
X-original-commit: c143e7d27ce25c88854fc3c2980f399add848cb3