As per your response, I believe the property on your work case is something like below,
When you upload the attachment, you will have the the Data-WorkAttach-File-Temp entry gets created. Once you finish the assignment, the temp file instance is committed to actual Data-WorkAttach-File table and linked to the case using Link-Attachment instance. Along with this the temp instance also removed. Due to this, you should be able to see the attachments linked to the case.
I see the above behaviour working fine in v25.1.2 and v24.1. Which version are you using?
You can find more details about this behaviour explained in another article regarding updation/deletion of attachments
Refer to the screenshots for reference.
Regards
JC