Confluence: How to remove Attachments during upload phase

[GBH4DSC]

Hi,
I want to remove specified attachments in confluence during or after an attachment upload performed by user.
The user should also get a popup notifier with a message.
Which confluence classes should I use?

I' ve tried using the AttachmentBatchUploadCompletedEvent, but there seems to be no possibility to create a popup notifier.

Moreover removing an attachment in AttachmentBatchUploadCompletedEvent though successfully done causes a
java.lang.NullPointerException at com.atlassian.confluence.pages.AttachmentDownloadPathCache.toKey(AttachmentDownloadPathCache.java:31)
at com.atlassian.confluence.pages.AttachmentDownloadPathCache.toKey(AttachmentDownloadPathCache.java:26)
at com.atlassian.confluence.pages.CachingAttachmentManager.invalidateCache(CachingAttachmentManager.java:147)
at com.atlassian.confluence.pages.CachingAttachmentManager.saveAttachments(CachingAttachmentManager.java:189)
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)

any help would be appreciated

[GBH4DSC]

Until now  no response - any suggestions?