CL_CRM_DOCUMENTS class contains all the methods you need to work with any document attachments in CRM. You should do some further searches here on the method GET_WITH_TABLE to see how that will work. In generaly you could build a new web service in your system that will act as a wrapper around code that will retrieve an attachment list for a given case. I have built before a standalone report to list all attachments related to a business transaction or business partner, and it's really not that hard, so wrapping it in a webservice should not be too much effort. If your partner has a specific WSDL they want you to generate then you probably need to use PI, otherwise if they dont' care then you can generate a new webservice from ABAP function/class and use that WSDL as what your partner will consume.
Take care,
Stephen