For instance, in the settings page for Tab Session Manager, you can click an upload button to upload JSON session files from your device to the addon. I looked up if UBlock Origin Lite (the re-implementation of UBlock Origin with Manifest v3) supports custom filter lists and all the results said no it can’t due to the limitations of Manifest V3. OTOH I found a StackOverflow post and Google Groups discussion saying its possible with the filesystem API.
I guess that kinda depends on what you mean by “upload”. If you simply mean if user can point the extension to load some local file eg. via `` then sure, that works just fine.
It sounds like the issue you mention regarding uBO Lite is not about if it can read a local file, but rather that it can’t use that data to update the active filter lists - perhaps because it already has too many dynamically inserted filter rules, but that’s just a guess.