How to make mp3 file surface in spotify local folder in IPhone

 1) Airdrop a mp3 file from Iphone to Macbook

2) Add mp3 file to Macbook music folder so music app can play it

4) Connect Iphone to Macbook 

5) Right click the song in music app and select menu "Add to device", select the connected Iphone

6) song can be played in Iphone music app

7) From spotify setting, turn on local folder, now spotify can play the song in music app

Note: IPhone can't open mp3 file directly, need to sysc from macbook or itune music app