Author of Msty here. Not yet but I am already working on the design for it to be added in very near future. I am happy to chat more with you to understand your needs and what you are looking in such apps. Please hop on the Discord if you don't mind :)
Some of my usecases would be summarizing a PDF report, analyzing json/csv data, upload a dev project to write a function or feature or build a UI, rename image files, categorize images, etc
Yes, GPT4All has RAG-like features. Basically you configure some directories and then have it load docs from whatever folders you have enabled for the model you're currently using. I haven't used it a ton, but I have used it to review long documents and it's worked well depending on the model.
Open-WebUI has support for doing that, it works using #tags for each document so you can ask questions about multiple specific documents.