I was also frustrated by being unable to change image filenames and the associated permalinks. I couldn’t find the “Edit more details” option, and there was no “Edit” button next to the Permalink field.
Then I changed my library view from “List view” to “Grid view.” (the two icons are at top left under the words “Media Library).
Once I was in Grid view, clicking on the image brought up the “Attachment Details” window with “Edit more details” visible at bottom right.
WHAT A SHAME that WordPress would have different interfaces depending on whether you are in grid view or list view!
But still, problem solved. Hope this is helpful to the community.
