undertale.sav is the save format used by console versions of UNDERTALE and UNDERTALE Mobile Mod, while "save files" (referring to files such as file0) are used by PC versions of UNDERTALE.
undertale.sav
file0
Select a undertale.sav file
Add multiple save files
Tip: To select multiple files, hold down the Ctrl or Shift key while selecting in Windows, or long-press a file then click on other files in Android's "Documents UI"