Timestamp from files should be offered as alternative when no exif time exists

[Penguin]

Some old phone cameras (such as Nokia 6300) don't save any exif metadata at all. However they still set the timestamp for the created image file on the storage device to the phone's current time.

When correlating pictures with gpx tracks I suggest that the file creation timestamp (or changed/modified timestamp under file systems that don't contain a creation timestamp such as ext3) could be offered as a fallback if no exif data is present in the image. Same goes for the "Use photo of an accurate clock" feature.

There are utilities out there to update images with this exif information manually but I reckon such a update in JOSM could be pretty easy to implement in any case.