Please describe the problem:
When downloading a video to my home directory and opening nautilus to display my home directory, the incomplete video files will often be thumbnailed as completely black (with the film strip at the edges). The only way to fix these thumbnails is to remove the ~/.thumbnails/normal directory, or possibly the specific thumbnail file in it. The thumbnailer should probably detect if the preview is empty and try to make a preview again later instead of having a permanent empty preview.

Steps to reproduce:
1. Download a video file to your home directory
2. Open nautilus to your home directory


Actual results:
Often, there is an empty thumbnail generated for an incomplete file, which remains even when the file is complete.

Expected results:
The thumbnailer doesn't generate empty previews, or updates the thumbnail later without having to manually delete the stored thumbnail

Does this happen every time?
Often, it's hard to reproduce exactly what sort of incomplete file can't be previewed

Other information: