use jpg as extension for all output files

src/main.rs

@@ -63,6 +63,7 @@ pub fn output_path(image_path: PathBuf) -> Result<PathBuf,String> {
         Some(cache) => match xdg_dirs.create_cache_directory(cache) {
             Ok(mut cache) => {
                 cache.push(image_path.strip_prefix("/").unwrap());
+                cache.add_extension("jpg");
                 Ok(cache)
             },
             Err(e) => Err(e.to_string()),