Centre-aligning the file block doesn't centre-align anything

[sarahmonster]

When centre-aligning the file block, it looks exactly the same in the editor as the left-aligned and default file block.

Default alignment:

Centre-aligned:

Left-aligned:

Right-aligned:

To Reproduce
Steps to reproduce the behavior:

1. Create a new post and add a file.
2. Centre-align block using the toolbar.
3. See left alignment!

Tested in Firefox Nightly on Mac. Seems to be properly centre-aligned in the frontend—looks as though the alignment class just isn't added in the editor.

[noisysocks]

Fixed by #8303.