a.js

//
  s3Uploader.content_type_for = function(file_input, file_name, content_type){
    content_type = content_type || 'application/octet-stream';

    try{
      content_type = jQuery(file_input).prop('files')[0]['type']
    } catch(e) {};

    if(!content_type){
      var parts = file_name.split('.');
      var base = parts.shift();
      var ext = parts.pop().toLowerCase().trim();

      switch(ext){
        case 'jpg':
        case 'jpeg':
          content_type = 'image/jpeg';
          break;
        case 'png':
          content_type = 'image/png';
          break;
        case 'gif':
          content_type = 'image/gif';
          break;
        case 'tiff':
        case 'tif':
          content_type = 'image/tiff';
          break;
      }
    };