Per the Reddit thread, FLAC files can be synced to an iOS device through iCloud Drive, then accessed through the new Files application, which will allow for local playback of the high-quality audio files directly on the device. If true, it would mark the first time that Apple has offered support for the popular FLAC file format on an iOS device.
FLAC is by far the most popular lossless standard
Until now, anyone hoping to have lossless, uncompressed audio on an iOS device had to use Apple’s own ALAC format, which — unlike FLAC — is supported by iTunes. And while on paper there’s no problem converting one lossless file type to another without losing quality (hence the term “lossless”), FLAC is by far the more popular standard, so it’s nice to see that Apple is finally supporting it in some fashion.
That said, the report indicates that FLAC files only work in the Files application, not in the Apple Music app. This makes sense, given that would likely require a more major revision to iTunes as well to support FLAC files to sync them. Still, even if Apple doesn’t offer that functionality in the future, at least audiophiles looking to play their FLAC collections on Apple’s devices may have some workaround when iOS 11 launches this fall.