While doing QA testing on Moodle 3.4, was unable to upload large files.
When I tried to upload a file larger than about 60 Mb, I got a communication error. This, even though the stated upload limit is 256 Mb. This happened whether uploading with Drag and Drop, or browsing. It also happened when loading file as regular file resource, as opposed to putting file in the folder.

When adding a large file via the upload file option, I got a circle spinning endlessly. With drop and drag, I got the error, "Alert: An error occurred whilst communicating with the server"

I was using the QA.moodle.net site with a wired connection.

Test steps:

1. Login as teacher, turn editing on and add a folder resource Verify that the File manager form element is present in the add folder resource form.
2. Click 'Add' and verify that the file picker is launched.
3. Select 'Upload a file', choose a large file (over 60 Mb) to upload and upload it.
4. Click 'Save changes' and verify that the files have been uploaded.