When using rectangle-mark-mode in an editable dired buffer, how can I edit the region?

[s1ohy]

Using rectangle-mark-mode[1] with rectangle-utils[2] I highlight a
region in an editable dired buffer and call the command
`upcase-initials-region`: http://gobin.io/WzjF but as you can see in
the minibuffer I get the message `Text is read-only`. Is there any way
I can edit the rectangle region?

I'm open to other ways of doing what I want as this upcases the 'F' in
'flac', which I don't want, but I'm still interested in my original
question as this could be used in other places.

[1] 
https://www.gnu.org/software/emacs/manual/html_node/emacs/Rectangles.html
[2] https://github.com/thierryvolpiatto/rectangle-utils