feat/add preview of image in workout if image file

If the uploaded file is an image, it is shown as a preview in the workout page.

frontend/www/scripts/workout.js

@@ -42,6 +42,15 @@ async function retrieveWorkout(id) {
             a.text = pathArray[pathArray.length - 1];
             a.className = "me-2";
             filesDiv.appendChild(a);
+
+            let isImage = ["jpg", "png", "gif", "jpeg"].includes(a.text.split(".")[1]);
+
+            if(isImage){ 
+                let img = document.createElement("img");
+                img.src = file.file;
+                img.width = "500";
+                filesDiv.appendChild(img);
+            }
         }
 
         // create exercises