The file path you quote is the "new" storage structure, used from sp16 onwards.
If you extract any servicepack distribution SDSK_#####.EXE archive from sp16 to now (latest sp39) in a temp folder, then you can find a doc subfolder with extra documentation for selected features and enhancements, such as this:
Depending on what you want to find in the FTP folder tree or in the rep_attachments table in the database, you can find corresponding records in the DB and relate them to specific files in the FTP tree.
Here's what I can tell you about rep_attachments's columns:
AHS_OID is unique (primary key), but it’s not consecutive.
AHS_ENT_OID is a reference to entity, for example
563019801 = the servicecall "object" > see table ifc_entities columns ent_oid and ent_name.
633318790267031 = the Email Servicecall "object" (Yes, SD can be used to construct emails, complete with attachments)
Other entity/object numbers for Workorders, Changes, Problem, etc. can be found with this query:
column ent_oid format 99999999999999999999
column ent_name format a30
select distinct ifc_entities.ent_oid, ifc_entities.ent_name from ifc_entities, rep_attachments where ahs_ent_oid=ent_oid;
AHS_BASENAME is the real name of the attachment, rather than the weird 0000000-0000-0000-0004-blablabla.
The rep_attachment table is the only place where this filename is stored, so this is what users see in the GUI.
AHS_ATT_OID is the reference to a specific [Servicecall | Email Sc | Historyline | Change | Problem| …] OID record which has (an) attachment(s).
It’s not unique, because Servicecall 1234 can have multiple attachments
Servicecall entity OID = 563019801 = AHS_ENT_OID = IFC_ENTITIES.ENT_OID