when i have a bunch of other, it turns out to be content that itunes isn't syncing right. and removing each synced content will fix it. i had this nasty problem where turning on itunes match sent my entire music library to other. and i had to turn off itunes match, manually delete any synced content and then turn it back on to clear it
i had a more recent problem where i would have like 20 gigs of photos, but the iPhone only sees 5 gigs with 15 gigs of other. i could never fix it, so i just switched to iCloud
the internet says that these are messages or temp files. i beg to differ , the times when i had more then a few gigs of other was always something related to a bad sync to my phone, be it photos, or music
i would argue that with iOS 9, theres not a shred of messages or attachments under messages that shows up as other.
