What

If you select a big list of files in the Media Library, you should be able to easily clear your selection and "de-select all" files you currently have highlighted.

Why

This is coupled with another Feature Request for downloading selected files from your library:
#70959

If you were to select a group of files to download, you should be able to quickly de-select them, and possibly select another group. You need a way to clear your selection quickly without clicking on every file individually again.

If the file list view feature request were implemented:
#70978

...then the icon view would need a way to clear it's selections. The checkmark would handle this on the list view.

How

On the icon view, there could be a button on the toolbar to clear your selection.