Added `File Handling' section

CODING.md

@@ -157,6 +157,20 @@ New Button
       * Client does not save the modified record.
       * Client stays on current page.
   
+File Handling
+-------------
+* No previous uploaded file present
+  1. User presses the Browse button
+    1. User selects file
+      1. File is uploaded to qfq immediately
+      1. Browse button gets disabled and hidden
+      1. File delete button is shown
+    1. User cancels file selection
+      1. no action
+* Previous uploaded file present
+  1. User deletes file
+    1. File delete button gets disabled and hidden
+    1. Browse button gets enabled and displayed
   
 Debug / Log / Errormessages
 ===========================