JS

function checkArtistDetails(form){

  var valid = true; //boolean
  var fileName = form.filePictureFileName.value.toLowerCase();  //Sets chosen filename to LowerCase.

 //Checks if txtArtist is empty
 if(form.txtArtist.value == "")
 {
    window.alert( "Please enter the artists name.");
    form.txtArtist.focus();
    valid = false ;
  }

   //Checks if a file is choosen
  if(fileName == "")
  {
    window.alert("Please choose a picture.");
    form.filePictureFileName.focus();
    valid = false;
  }

/*
#############################
HERE'S THE PROBLEM.
#############################
*/
     //Checks if image-extension is correct
  if(fileName.lastIndexOf(".jpg") == -1 || fileName.lastIndexOf(".png") == -1)
  {
     window.alert("Only '.jpg' and '.png' file types are allowed.");
     valid = false;
  }

  else{
  return valid;}
}

//resets ArtistName-field
function resetArtistName(form){
form.txtArtist.value="";
}