SAP CRM: How order attachments are stored
First off, attachments are completely independent from the rest of the transaction. As soon as you add the attachment, it's saved on the DB - no need to save the transaction.
If you're looking for standard source code how to add attachments to an object, set a breakpoint in CL_CRM_DOCUMENTS=>CREATE_WITH_TABLE and upload an attachment. (CL_CRM_BSP_CM_CONTROL=>CREATE_DOCUMENT contains some useful methods to fill the parameters.)
Once the attachment is saved, you can find it in the following tables: