[ZEPPELIN-6146] Replace 'application/zpln' with '.zpln' in 'accept' attribute of file importer

### What is this PR for?
Currently, in new UI, it's not that easy to import notes fron .zpln files, since by default file importer only looks for .json files. The bug was reproduced in both Win10 and MacOs

### What type of PR is it?
Bug Fix

### What is the Jira issue?
https://issues.apache.org/jira/browse/ZEPPELIN-6146

Closes #4895 from Charlie91/change-fileimporter-accept-rule.

Signed-off-by: Philipp Dallig <[email protected]>
(cherry picked from commit 80c8825)
Signed-off-by: Philipp Dallig <[email protected]>

Author: Charlie91

zeppelin-web-angular/src/app/share/note-import/note-import.component.html

@@ -21,7 +21,7 @@
 
 <nz-tabset>
   <nz-tab nzTitle="Import From JSON File">
-    <nz-upload nzType="drag" [nzBeforeUpload]="beforeUpload" nzAccept="application/json,application/zpln">
+    <nz-upload nzType="drag" [nzBeforeUpload]="beforeUpload" nzAccept="application/json, .zpln">
       <p class="ant-upload-drag-icon">
         <i nz-icon nzType="inbox"></i>
       </p>