Hi,
I finally got around to recreating the error. Easy enough, it happens with any file which has a .txt extension. As I mentioned before, if I re-extension the file .something-else it is fine. Just .txt files fail.
So, your questions (thanks for the response).
Yes, I am using SSL, and using the Flash uploader. But (A) I have no problems using this with any other file and (B) I fixed the Flash error by making sure that my cert (which is issued by Comodo) contains the full chain - this seems to fix that problem.
I checked the alfresco.log. It has not been modified since 20th December (two months ago), there is no record of any error relating to this problem. I did search the whole machine for instances of the log, there is only one (/opt/alfresco/alfresco.log).
Not helpful I know. Does anyone have any other ideas?
Andy