UTL_MAIL does not work like that - you pointing to an external file and telling it to grab it and attached it to your e-mail.
If you refer to the manual, the attachment parameter specifies it must contain the contents of the file.
So your code needs to open the file. Your code needs to read the contents of the file. Your code needs to close the file. Your code needs to supply the contents text to UTL_MAIL via the attachment parameter.
And as there is only a single attachment parameter, only a single file's contents can be supplied as attachment.
Multiple attachments? Then you need to do what UTL_MAIL does. You need to write code that creates a valid e-mail Mime body. The Mime body must allow for multiple attachment. The Mime body needs to be delivered as an e-mail via the UTL_SMTP package.