To add attachments to a Sales Order, go into the Sale, go to the Attachments subtab.

Click Choose and select all the files to upload.
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After attaching the attachment, you can edit the name and name the file to your needs.

Note: Only .csv, .xls, .xlsx .xlx, .odt, .doc, .docx, .jpg, .jpeg, .png, and .pdf are supported. File size must not exceede 5MB.