Hi Ethan
A. Is this migrated DB from M76 -
if Yes you will need to update the path for cos:doclinks in DOCINFO table before you can open them from Work Order or any other record in MAS.
For example in M76 URL Name D:\DOCLINKS\Attachment\image001.png
that should be updated to cos:doclinks/image001.png
Important to note MAS on S3 doesn't support files in Child folder so you also need to move all child files to root of S3.
B. If this is not migrated from M76 and fresh setup then you need to attach documents with different records like Work Order or Asset in Maximo before you can access it.
When you will attach document, it will get saved on root of S3 that you have configured.
Thanks