Thumbnails falsely show not found under certain conditions

[oddnoc]

Publishing files can sometimes cause thumbnails to show not found (turning red). Refreshing the admin reveals that the files are actually there. The video shows this happening. This is a production site whose assets are served over NFS, so our working hypothesis is that there is a short timeout that expires during NFS latency. Could also be related to #1042.

In the video, the user publishes a file, then publishes a second file. When the second file is published, the thumbnails of the first and second files both turn red. The user then refreshes the browser and the thumbnails show up correctly.