image step validation

app/javascript/submission_form/dropzone.vue

@@ -95,7 +95,19 @@ export default {
   },
   methods: {
     onDropFiles (e) {
-      this.uploadFiles(e.dataTransfer.files)
+      const files = Array.from(e.dataTransfer.files).filter((f) => {
+        if (this.accept === 'image/*') {
+          return f.type.startsWith('image')
+        } else {
+          return true
+        }
+      })
+
+      if (this.accept === 'image/*' && !files.length) {
+        alert(this.t('please_upload_an_image_file'))
+      } else {
+        this.uploadFiles(files)
+      }
     },
     onSelectFiles (e) {
       e.preventDefault()