After uploading a new attachment all existing attachments are scanned again.

[mofterdinger]

After uploading a new attachment all existing attachments are scanned again. Here an example:

In this example, 2 attachments are already uploaded and scanned. Then, a third attachment is uploaded and all attachments are scanned again. This needs to be avoided, because the first two attachments were already scanned and not modified during the upload of the 3. attachments.
While the attachments are scanned, they can't be opened until the scan has finished and the content is clean.

[mofterdinger]

This issue is fixed with #486.