feat: guess format from magic numbers as well

this allows opening files with extensions that don't match the filetype

src/utils.rs

@@ -7,7 +7,7 @@ use rfd::FileDialog;
 pub fn load_image(path: impl AsRef<Path>) -> Result<RgbaImage, String> {
     let path = path.as_ref();
 
-    let image_reader = match ImageReader::open(path) {
+    let image_reader = match ImageReader::open(path).and_then(|r| r.with_guessed_format()) {
         Ok(r) => r,
         Err(e) => {
             return Err(format!(