minor

client/style/style.scss

@@ -418,3 +418,7 @@ input[type=file] {
 	position: absolute;
 	z-index: -1;
 }
+
+.was-validated input[type=file]:invalid + button {
+	border-color: $red;
+}

client/view/controls/file.js

@@ -7,6 +7,7 @@ class FileControl extends BaseControl {
         this.fileButton = frappe.ui.create('button', {
             className: 'btn btn-outline-secondary btn-block',
             inside: this.getInputParent(),
+            type: 'button',
             textContent: 'Choose a file...',
             onclick: () => {
                 this.input.click();