There are some situations where a file might be visible when listed, but aren't actually fully-present on the file system. The most notable is when files are in OneDrive and are currently offline.

The problem with this is that if you browse to a folder containing offline files while in thumbnail view, the process of creating thumbnails will necessarily require the downloading of all files in that folder. This can take a very long time, and be quite confusing, if a large number of files and/or if your internet connection is slow.

This patch checks for this status when getting file attributes. Some offline systems use FILE_ATTRIBUTE_OFFLINE to indicate this. OneDrive also used this attribute until 2019 and then starting using FILE_ATTRIBUTE_RECALL_ON_DATA_ACCESS. This patch treats them both as the same as our internal FILE_ATTR_OFFLINE.

With that attribute set this patch just makes such files not available for thumbnails.

For example, I have two images in my OneDrive folder. One shows a green checkmark because it is online and available. The other shows a cloud because it is not.

Viewing in the Blender File Browser they will look like this, with the offline one getting a generic image icon. However a small icon is added to indicate that it is an offline file.

But after viewing, the offline file is still offline.

@Ray Molenkamp (LazyDodo) - Not sure how to tell how old this FILE_ATTRIBUTE_RECALL_ON_DATA_ACCESS define is and in our current minimum Windows API version.