At this time Identity Governance and Identity Portal approach uploading files in two different ways. Currently Identity Governance validates the file at a UI level, while Identity Portal validates at a web service level.
So what this means for uploading attachments is Identity Portal can handle a larger load than Identity Governance. To keep the two in sync with each other we would recommend following Identity Governance standards.
We recommend uploading no more than 100 mb files in the Identity Governance UI and you can only attach one file at a time. Testing can be conducted on Identity Portal by uploading a larger size and see the behavior on your environment if necessary.
You can also upload multiple files that in total exceed 100 mb but the single file being uploaded must be less than 100 mb.