No, you can't rename an attachment after you uploaded it. You need to detach the file, rename it and upload it again.
Also note that GMail can sometimes renames your attachments. Here's an useful quote from the official documentation:
Some attachments that include non-English characters in the filename may be renamed 'Gmail,' or the name may be truncated -- this happens when Google doesn't recognized the encoding used to name the attachment. Our engineers are working on a solution, but in the meantime, we suggest using ASCII characters so we can always recognize the names of your attachments.
If your attachment name was renamed 'noname,' you may be able to display the file by clicking on the View as text link that appears next to the attachment. If that link does not appear, we were unable to display the file. This is a known issue, and our engineers are working to resolve it as quickly as possible.