save item fails on thumbnail upload if file disappears from filesystem

[ch604]

steps to reproduce:

1. add an item to a collection without a thumbnail
2. edit the item
3. add a thumbnail from your local computer
4. wait for the small preview to appear indicating that koillection has recognized the file
5. move, rename, or delete the file from the local computer
6. attempt to save the item

expected behavior: since koillection has previewed the file, it is expected that it has already stored the data, or it should indicate that it no longer has access to the file in question, remove the thumbnail preview and associated content, and save the remainder of the edit.

actual behavior: the save stalls, and no error message is given. refreshing the page or navigating away undoes all edits.