Sorry to keep whining to you guys :)
After upgrading from 2.5.3 to 2.7.0 and then to 2.7.1 in the last few days, I've discovered that creating new content and editing existing content are both failing with the complaint that "no file was selected for upload". This happens at every instance of the native "file" datatype, and it happens in spite of none of them being marked as required. A quick scan of the DB confirms that the "not required" setting is also correctly being stored in the exp_channel_fields table.
Oddly, this only happens when the attribute is intended to receive an image. Channels which can take an optional generic file upload can be entered without problems without an attachment being present.
Nothing obvious in the console, nothing at all in the log files.
Anybody seen this before?