Did you transfer your files in binary mode? if not, re-upload the attachment files using binary mode. Ask Google for example if you do not know, how to do it with your favourite ftp-program.
I can safely take them and pass them from the PC in binary mode ftp?
That should be enough, yes. If this does not work, however, you'll at least need to download the original attachments again. But then in Binary mode of course.
You can first try to open it on your PC with the "Open with..." dialog and select your normal photo viewer (with an attachment, where you know it's an image). If this works you don't have to download them again. If it does not work you have to download them again.