Currently removeAttachmend Web Service method does not support removal of the attachments that are not related to any object. If you try to do that in stdlog, you will get the following error in "api.spl 7899 Error deleting where clause 'attmnt = 400258': Unknown name".
Nevertheless create attachment method allow to upload file without attaching it to an object, this can be achieved by passing Null value for objectHandle.
So the idea is to add such functionality to removeAttachmend method.
This improvement would be important for implementation of the following use case:
At the incident registration moment it is not possible to relate attachment with the ticket since there is no ticket registered at that time. However, it is possible to upload attachments without relating them to any object, and after ticket is registered, these attachments can be related with the ticket by using createLrelRelationships method. Everything works well except one case, when user uploads some attachments and decides to cancel ticket registration. In this case there is no way to cleanup attachments from SDM.