How to “yield put” in redux-saga within a callback?

One possible solution, as you already mentioned, is to use channels. Here is an example that should work in your case:

import { channel } from 'redux-saga'
import { put, take } from 'redux-saga/effects'

const downloadFileChannel = channel()

export function* loadFile(id) {
  ...
  const download = RNFS.downloadFile({
     ...
     // push `S_PROGRESS` action into channel on each progress event
     progress: (progress) => downloadFileChannel.put({
       type: ACTIONS.S_PROGRESS,
       progress,
     }),
  })
  ...
}

export function* watchDownloadFileChannel() {
  while (true) {
    const action = yield take(downloadFileChannel)
    yield put(action)
  }
}

The idea here is that we will push a S_PROGRESS action on the channel for each progress event that is emitted from RNFS.downloadFile.

We also have to start another saga function that is listening to each pushed action in a while loop (watchDownloadFileChannel). Everytime an action has been taken from the channel, we use the normal yield put to tell redux-saga that this action should be dispatched.

I hope this answer will help you.