diff --git a/app/controllers/concerns/diff_for_path.rb b/app/controllers/concerns/diff_for_path.rb
index aeec3009f15..1efa9fe060f 100644
--- a/app/controllers/concerns/diff_for_path.rb
+++ b/app/controllers/concerns/diff_for_path.rb
@@ -3,7 +3,7 @@ module DiffForPath
 
   def render_diff_for_path(diffs)
     diff_file = diffs.diff_files.find do |diff|
-      diff.old_path == params[:old_path] && diff.new_path == params[:new_path]
+      diff.file_identifier == params[:file_identifier]
     end
 
     return render_404 unless diff_file
diff --git a/app/views/projects/diffs/_content.html.haml b/app/views/projects/diffs/_content.html.haml
index 779c8ea0104..c3d2f80544b 100644
--- a/app/views/projects/diffs/_content.html.haml
+++ b/app/views/projects/diffs/_content.html.haml
@@ -9,7 +9,7 @@
     - if !project.repository.diffable?(blob)
       .nothing-here-block This diff was suppressed by a .gitattributes entry.
     - elsif diff_file.collapsed?
-      - url = url_for(params.merge(action: :diff_for_path, old_path: diff_file.old_path, new_path: diff_file.new_path))
+      - url = url_for(params.merge(action: :diff_for_path, old_path: diff_file.old_path, new_path: diff_file.new_path, file_identifier: diff_file.file_identifier))
       .nothing-here-block.diff-collapsed{data: { diff_for_path: url } }
         This diff is collapsed.
         %a.click-to-expand
diff --git a/changelogs/unreleased/adam-fix-collapsed-diff-symlink-file-conversion.yml b/changelogs/unreleased/adam-fix-collapsed-diff-symlink-file-conversion.yml
new file mode 100644
index 00000000000..c83558f33d1
--- /dev/null
+++ b/changelogs/unreleased/adam-fix-collapsed-diff-symlink-file-conversion.yml
@@ -0,0 +1,4 @@
+---
+title: Fix expanding a collapsed diff when converting a symlink to a regular file
+merge_request: 6953
+author:
diff --git a/lib/gitlab/diff/file.rb b/lib/gitlab/diff/file.rb
index ce85e5e0123..5110bfbf898 100644
--- a/lib/gitlab/diff/file.rb
+++ b/lib/gitlab/diff/file.rb
@@ -126,7 +126,7 @@ module Gitlab
         repository.blob_at(commit.id, file_path)
       end
 
-      def cache_key
+      def file_identifier
         "#{file_path}-#{new_file}-#{deleted_file}-#{renamed_file}"
       end
     end
diff --git a/lib/gitlab/diff/file_collection/merge_request_diff.rb b/lib/gitlab/diff/file_collection/merge_request_diff.rb
index dc4d47c878b..fe7adb7bed6 100644
--- a/lib/gitlab/diff/file_collection/merge_request_diff.rb
+++ b/lib/gitlab/diff/file_collection/merge_request_diff.rb
@@ -39,7 +39,7 @@ module Gitlab
         # hashes that represent serialized diff lines.
         #
         def cache_highlight!(diff_file)
-          item_key = diff_file.cache_key
+          item_key = diff_file.file_identifier
 
           if highlight_cache[item_key]
             highlight_diff_file_from_cache!(diff_file, highlight_cache[item_key])
diff --git a/spec/features/expand_collapse_diffs_spec.rb b/spec/features/expand_collapse_diffs_spec.rb
index 6c938bdead8..3934c936f20 100644
--- a/spec/features/expand_collapse_diffs_spec.rb
+++ b/spec/features/expand_collapse_diffs_spec.rb
@@ -182,6 +182,20 @@ feature 'Expand and collapse diffs', js: true, feature: true do
         end
       end
     end
+
+    context 'expanding a diff when symlink was converted to a regular file' do
+      let(:branch) { 'symlink-expand-diff' }
+
+      it 'shows the content of the regular file' do
+        expect(page).to have_content('This diff is collapsed')
+        expect(page).to have_no_content('No longer a symlink')
+
+        find('.click-to-expand').click
+        wait_for_ajax
+
+        expect(page).to have_content('No longer a symlink')
+      end
+    end
   end
 
   context 'visiting a commit without collapsed diffs' do
diff --git a/spec/support/test_env.rb b/spec/support/test_env.rb
index 778e665500d..103f7542286 100644
--- a/spec/support/test_env.rb
+++ b/spec/support/test_env.rb
@@ -23,6 +23,7 @@ module TestEnv
     'binary-encoding'                    => '7b1cf43',
     'gitattributes'                      => '5a62481',
     'expand-collapse-diffs'              => '4842455',
+    'symlink-expand-diff'                => '81e6355',
     'expand-collapse-files'              => '025db92',
     'expand-collapse-lines'              => '238e82d',
     'video'                              => '8879059',