org.el: Preserve case for link subgroups from `org-file-apps'

* lisp/org.el (org-open-file): Avoid matching of `org-file-apps' records
against the link converted to downcase since it caused incorrect
substitutions to the command.

Consider the following entry

    (add-to-list
     'org-file-apps
     '("\\.PDF\\(?:\\.[gx]z\\|\\.bz2\\)?::\\(#[^:]*\\)::\\(.+\\)\\'"
      . "okular --find %2 -- %s%1"))

and the link
<file:///usr/share/doc/bash/bashref.pdf::#Redirections::before>
Without the patch

    okular --find before -- /usr/share/doc/bash/bashref.pdf\#redirections

command is executed and the application can not resolve internal
cross-reference anchor.

In https://list.orgmode.org/4B51D104.9090502@jboecker.de/T/#u
https://list.orgmode.org/k2jfb2eb6811004041733zf176e0aq8367924746db81f5@mail.gmail.com/T/#u
the purpose of `dlink' is not clarified, so I assume that the only
purpose is to allow matching file suffixes, e.g. ".pdf" vs ".PDF".

lisp/org.el

@@ -8134,7 +8134,6 @@ If the file does not exist, throw an error."
 	 (link (cond (line (concat file "::" (number-to-string line)))
 		     (search (concat file "::" search))
 		     (t file)))
-	 (dlink (downcase link))
 	 (ext
 	  (and (string-match "\\`.*?\\.\\([a-zA-Z0-9]+\\(\\.gz\\)?\\)\\'" dfile)
 	       (match-string 1 dfile)))
@@ -8159,8 +8158,9 @@ If the file does not exist, throw an error."
 		    ;; First, try matching against apps-dlink if we
 		    ;; get a match here, store the match data for
 		    ;; later.
-		    (let ((match (assoc-default dlink apps-dlink
-						'string-match)))
+		    (let* ((case-fold-search t)
+                           (match (assoc-default link apps-dlink
+                                                 'string-match)))
 		      (if match
 			  (progn (setq link-match-data (match-data))
 				 match)
@@ -8191,7 +8191,7 @@ If the file does not exist, throw an error."
       (user-error "No such file: %s" file))
     (cond
      ((org-string-nw-p cmd)
-      (setq cmd (org--open-file-format-command cmd file dlink link-match-data))
+      (setq cmd (org--open-file-format-command cmd file link link-match-data))
 
       (save-window-excursion
 	(message "Running %s...done" cmd)