Just verify that the "Visible to Supplier" option is selected while uploading a document.
When we click on Add Attachment button, you should see a File option to browse the document from your system and there is also a checkbox "Visible to Supplier". If this is checked, attachment should go to the supplier otherwise not. Kindly verify if it is checked. There are certain parameters related to attachments which are defined in Parameters.table file like AllowedAttachmentExtensions where you can define a list of extensions you can allow a user like .doc, .xls etc; MaxAttachmentSize. These parameters will go you some more control on attachments.