Fix extension keymap context for single file worktree (#40425)

Closes #40353

Release Notes:

- Fixed `extension` in keymap context being empty for single file
worktree

Co-authored-by: Cole Miller <cole@zed.dev>

crates/editor/src/editor.rs

@@ -2523,12 +2523,15 @@ impl Editor {
         }
 
         if let Some(singleton_buffer) = self.buffer.read(cx).as_singleton() {
-            if let Some(extension) = singleton_buffer
-                .read(cx)
-                .file()
-                .and_then(|file| file.path().extension())
-            {
-                key_context.set("extension", extension.to_string());
+            if let Some(extension) = singleton_buffer.read(cx).file().and_then(|file| {
+                Some(
+                    file.full_path(cx)
+                        .extension()?
+                        .to_string_lossy()
+                        .into_owned(),
+                )
+            }) {
+                key_context.set("extension", extension);
             }
         } else {
             key_context.add("multibuffer");