Thanks for your message - the UI you see there is used when you are trying to bulk import Thumbnails to many entities (in this example we will use Shots). Bulk import is used to match many selected Thumbnail images to existing Shots in Shotgun. If the name of the Thumbnail matches the name of the
Shot Code then Shotgun knows where each Thumbnail belongs.
If you want to upload only one thumbnail to one Shot, the thumbnail image name does not need to match the corresponding
Shot Code. Click the “Upload Thumbnail” link in the Thumbnail field and upload it individually by selecting “Choose File” in the image below.
If you select “Open Bulk Import” then you can upload many images to many Shots’ Thumbnail fields, but you need to be able to match that image filename to the
Shot Code (or
Description if that makes more sense for your workflow, but
Shot Code is the likely choice). You can see in the image below that the image filename is matching the Shot name.