refactor: Remove add_dummy_extension() helper (#682)

The helper method is redundant (_introduced as a [bug fix in Mar
2022](#306), there is a much
simpler approach to append the extension.

-
[`as_mut_os_string()`](https://doc.rust-lang.org/std/path/struct.Path.html#method.as_mut_os_str)
has been available since Rust `1.70.0`, matching the MSRV of this crate
since the introduction of
[`OnceLock`](https://doc.rust-lang.org/std/sync/struct.OnceLock.html)
[in Oct 2024](#515) (_`0.14.1`
release_).
- Additionally revised the comment and included a conditional statement
to better communicate the intent for why this placeholder is necessary.

Author: epage

src/file/source/file.rs

@@ -59,9 +59,13 @@ impl FileSourceFile {
                 )))
             };
         }
-        // Adding a dummy extension will make sure we will not override secondary extensions, i.e. "file.local"
-        // This will make the following set_extension function calls to append the extension.
-        let mut filename = add_dummy_extension(filename);
+
+        let mut filename = filename;
+        // Preserve any extension-like text within the provided file stem by appending a fake extension
+        // which will be replaced by `set_extension()` calls (e.g.  `file.local.placeholder` => `file.local.json`)
+        if filename.extension().is_some() {
+            filename.as_mut_os_string().push(".placeholder");
+        }
 
         match format_hint {
             Some(format) => {
@@ -134,18 +138,3 @@ where
         })
     }
 }
-
-fn add_dummy_extension(mut filename: PathBuf) -> PathBuf {
-    match filename.extension() {
-        Some(extension) => {
-            let mut ext = extension.to_os_string();
-            ext.push(".");
-            ext.push("dummy");
-            filename.set_extension(ext);
-        }
-        None => {
-            filename.set_extension("dummy");
-        }
-    }
-    filename
-}