My most-used means of file transfer is "e-mailed zip file", and generally this works well - until, of course, someone can't open a zip file on their phone. Which I'd assume is most phone users. The phone doesn't need an unzip utility, because a phone user expects a link to some cloud service (Google Photos, etc.) to achieve the same thing. The content doesn't live on the device; it's just a dumb terminal.
Taken to its extreme, I recently had to downmix several HD videos taken on a phone onto a DVD, and _physically mail it overseas_, because even the cloud option wasn't open to the user.