Steps to reproduce:

• click the "Create Issue" button and add an attachment
• open a new tab or window in the same browser (while keeping the "Create Issue" dialog open)
• click the "Create Issue" button and close the dialog, after it appeared
• change back to the first dialog
• click the "configure fields" and switch the mode from all to custom and back again
• [BUG] the attachment that was uploaded is now vanished

To reproduce the edit issue case, simply edit an issue, add an attachment and edit the same issue in another tab or window.