Draft: Do not modify filename of directly opened attachments

Attachments that are opened using “Open directly” are saved to a temporary file in the OS-defined directory for temporary files. They are currently saved there with a modified filename to ensure uniqueness (to not overwrite other files there). This filename modification is a bit inconvenient for a user that wants to save the file after having opened it by this route, as they most likely will remove the prepended string. This patch saves the file in a directory with a unique name, without changing the name of the file itself (or naming it ‘anonymous_file’ if the filename is empty), thus removing the inconvenience.