Replace hard coded impl in getImageMimeType

Signed-off-by: Yukai Huang <yukaihuangtw@gmail.com>

lib/utils.js

@@ -2,29 +2,14 @@
 const fs = require('fs')
 const path = require('path')
 const bodyParser = require('body-parser')
+const mime = require('mime-types')
 
 exports.isSQLite = function isSQLite (sequelize) {
   return sequelize.options.dialect === 'sqlite'
 }
 
 exports.getImageMimeType = function getImageMimeType (imagePath) {
-  const fileExtension = /[^.]+$/.exec(imagePath)
-
-  switch (fileExtension[0]) {
-    case 'bmp':
-      return 'image/bmp'
-    case 'gif':
-      return 'image/gif'
-    case 'jpg':
-    case 'jpeg':
-      return 'image/jpeg'
-    case 'png':
-      return 'image/png'
-    case 'tiff':
-      return 'image/tiff'
-    default:
-      return undefined
-  }
+  return mime.contentType(path.extname(imagePath))
 }
 
 exports.isRevealTheme = function isRevealTheme (theme) {