Fix AttachmentModel::openAttachment

• Fix the logic to check for valid / invalid filenames.
• Put files in a unique temporary dir to avoid naming conflicts

when files are stored in the same directory multiple times.

• QTemporaryDir also creates the directory which the old code

did not.

• Use QUrl::fromLocalFile to construct a correct Windows file

URL.

Tested both with valid and invalid file names. A remaining issue
is that on Windows the temporary dir will be left around.

• GnuPG-Bug-Id: T6760