You can rename attachments in bulk without downloading them by using the Rename Attachments with Airtable Field automation.
This automation preserves the original file extension (i.e. .jpeg or .pdf) automatically. You will only need to specify the actual name of the file, not the extension. For example: To rename a file from Test.pdf to New.pdf, the Airtable field you use to define the new filename would just contain New.
To set this up, simply follow the steps below:
Select the field that contains the attachments that you want to rename.
Choose the field containing the desired name for the attachment (You can opt for any formula, long text, rich text, or single text field type).
Optional: When selecting the Prefix field to attachment name option, the content of the Airtable field (#3) will be inserted as a prefix to the existing name of the attachment.