Taking you example:

![438...75.png](:/8c...5ce1)

438...75.png is the original name and extension of the file that was attached (in this case it's not much different from the id but it can be something more meaningful).

8c...5ce1 is the id of the resource corresponding to the file, it's used as a key in DB and for links, among other things.

As you have already figured out the name of the corresponding file in Joplin's storage is
.resources/<id>.<ext>

The prefix :/ is used to identify Joplin internal links.