just had a try and it worked well. Could you please check "File Types" in IDEA? Open "Settings" -> type "file types" to search "File Types" setting -> Check "Protocol Buffers file" in "Recognized File Types", make sure "*.proto" is in "Registered Patterns”.
Stack Overflow里面提到的解决方案：https://stackoverflow.com/questions/19577878/intellij-proto-support