Comment 5 for bug 1069225

For raw file previews, the Preview thumbnail generator used by the Dash doesn't handle all types of raw files, so the lens is using the "*_shotwell.jpg" file Shotwell generates in the same folder as the raw file. A file it generates only when the raw is double clicked in Shotwell.

One way to fix it would be to use the largest available thumbnail (thumbs360/) in the Preview when the file is not supported, which is obviously quite hard to check, and could also introduce the need to maintain a hardcoded list of raw file extensions.
Ideally, we could ask the Dash devs for a Unity method to check if an extension is supported in the Dash (results and previews).

I will file a Unity bug for this and raise the subject next week at UDS.