You can rename an attachment in the workflow before the attachment is added in an email with the advanced email tool component, Rename attachment.

The component has these parameters:

1. Current attachment name
   This is the existing attachment, that has previously been loaded in the current workflow. You can either specify a fixed value or click the T-icon to select the attachment via an XPath expression.
2. Rename to
   Here you specify the name, that the attachment should be renamed into. You can either specify a fixed value or click the T-icon to select the attachment via an XPath expression.

An example of use can be found here.

If you see an attachment name when you receive the email like this:

Then you are probably using an invalid character in the attachment name. There is no standard as to which characters, that are allowed so you need to test for invalid characters and e.g. replace characters not found in the lists: a-z, A-Z and 0-9. On one mail server the dash (minus) character caused such an attachment name.