Does anyone know how I can open an attachment from a work item without having to download it first?
When I double click the attachment, the explorer is opened and the file is downloaded and only then I can open it.
You can't as far as I'm aware as the IDE will get the attachment from the TFS server using a web service and thus opens a browser and uses IE for this.
Please see this related item for more info: Open TFS Work Item Attachment in Image Viewer not Web Browser