Thanks for the reply Nigel.
There was no corresponding error message in the log file.
The file is a jpeg image, not a pdf file.
I ended up producing a thumbnail image by hand (100px wide) and uploading it to the server. This worked fine, but I still don’t understand what prevented Kiwitrees from generating the thumbnail. The original image was only 175KB so even if it had been highly compressed I don’t see how uncompressing it would cause a problem.