Instead of the for/of loop with await in it, you can use filelist.map() with an async callback.

Since .map() just blindly iterates the array without waiting for any returned promises, it will return an array of promises from all the async callbacks that .map() called. Those promises will initially be unfulfilled and all the fetch() operations will be "in-flight" at the same time. You can then use await Promise.all(...) on that returned array of promises to know when they are all done:

await Promise.all(filelist.map(async fileindex => {
    let dcmFilename = `slice_${fileindex}.dcm`
    slices.push({ index: fileindex, filename: dcmFilename })
    fd.append(dcmFilename, files[fileindex], dcmFilename);
    fd.append('slices', JSON.stringify(slices));
    loopcount += 1
    if (filecount == 24 || loopcount == filelist.length) {

        if (cursorPos !== false) {
            let scaleFactor = Tegaki.scaleFactor;
            cursorPos.x = parseInt(cursorPos.x / scaleFactor);
            cursorPos.y = parseInt(cursorPos.y / scaleFactor);
            fd.append('x', cursorPos.x);
            fd.append('y', cursorPos.y)
        }

        // Get layer id from index
        if (index !== -1) {
            type = this.layerTypes[index]['id']
        }
        // switch mode from heuristic to pytorch vertebrae for vertebral bone
        if (type === 'vertebral-bone') {
            mode = 'PyTorch-Vertebrae'
        }

        // Post to endpoint
        let domain = window.location.origin.replace(':8080', ':5000')

        let list = await fetch(`${domain}/segment?mode=${mode}&type=${type}`, { method: 'POST', body: fd })
        let result = await list.json()
        // do something with result
        // finish then continue the loop and create new body and send a new request
    }
}));

Note #1: Since you are now running multiple fetch operations in parallel, any code processing their results must not share variables. You have a few variables in this code that do not show their declarations. Those declarations should probably be inside this loop with let or const so there's a new variable for each iteration of the loop and you aren't sharing variables across iterations unless the variable is explicitly supposed to be accumulated across iterations. Suspicious variables in this loop with no local declaration that may be affected by parallel operation are filecount, loopcount, cursorPos, index, mode fd and type. You don't show the whole execution context here so we can't see enough to make a complete recommendation on these.

Note #2: The code that contains this if (filecount == 24 || loopcount == filelist.length) looks like it may be prone to problems. If you're trying to run some code when all the iterations are done, it would be better to run that code after the Promise.all() than trying to detect when your last iteration is done. Remember, your iterations will not necessarily proceed in order now because you're running them in parallel. They will launch in order, but not necessarily complete in order.