I used to be able to generate an attachment match table such as the below:
My feature class to which attachments was linked only had EC0020 in it, but it still managed to search through my folder and generate my attachment match table based on all the documents it found with a EC0020 in it. However, I am trying to do this now, and I keep getting an error saying my attachment table is empty. I have checked and when I make my document name EC0020, then it picks it up, but obviously you cannot have 4 documents with the exact same name. Can anyone please assist?
I have also tried on the latest version of ArcGIS Pro (10.9.1), I do not think what you want there (approximate match) is being supported anyway through the "Generate Attachment Match Table" tool. You may also have read the documentation, it only has something around various file extensions matching, but not to the file name.
I actually think it quite makes sense to only do the "exact match", imagining you also have "EC00201" which may be a total different object, but it would be matched in your expected case.
To achieve what you want, I think you need to go for Python to use some regular expressions to do the matching as you want.
It may not help, but good luck with your further exploration.