export RAF files from web based light room

Which app are you trying to export the RAF from? You mention "web based Lightroom" in your post title, so do you mean the LrWeb app which runs in a browser window (lightroom.adobe.com), or the Lightroom app which runs on the desktop or mobile devices?

In all these apps you can only download/export the original raw file (RAF or other proprietary raws) if the original file has been uploaded to the cloud. If you originally synced the RAF from Lightroom Classic you will only have a smart preview in the cloud, which is 2560px on the long edge. In that situation you cannot export or download the original because it doesn't exist in the cloud, so you would be restricted to a jpeg export of maximum 2560px long edge.

If you do have the original raw file in the cloud, your options vary depending on which app you are using. LrWeb would let you download the raw file, but without edits....but only the owner can do this via their own LrWeb portal, but if trying to do this from a shared album portal only Jpeg download is possible. Lightroom Desktop would let you export the original WITH edits, Lightroom Mobile would NOT let you export the original with edits, but you could export as DNG, which would retain the raw file format and include edits.