Detect MIME type of uploaded file in Ruby

Question

Is there a bullet proof way to detect MIME type of uploaded file in Ruby or Ruby on Rails? I'm uploading JPEGs and PNGs using SWFupload and content_type is always "application/octet-stream"

Answer 1

The ruby-filemagic gem will do it:

require 'filemagic'

puts FileMagic.new(FileMagic::MAGIC_MIME).file(__FILE__)
# => text/x-ruby; charset=us-ascii

This gem does not look at the file extension at all. It reads a bit of the file contents and uses that to guess the file's type.

Answer 2

In Ruby on Rails you can do:

MIME::Types.type_for("filename.gif").first.content_type # => "image/gif"

Answer 3

You can use this reliable method base on the magic header of the file :

def get_image_extension(local_file_path)
  png = Regexp.new("\x89PNG".force_encoding("binary"))
  jpg = Regexp.new("\xff\xd8\xff\xe0\x00\x10JFIF".force_encoding("binary"))
  jpg2 = Regexp.new("\xff\xd8\xff\xe1(.*){2}Exif".force_encoding("binary"))
  case IO.read(local_file_path, 10)
  when /^GIF8/
    'gif'
  when /^#{png}/
    'png'
  when /^#{jpg}/
    'jpg'
  when /^#{jpg2}/
    'jpg'
  else
    mime_type = `file #{local_file_path} --mime-type`.gsub("\n", '') # Works on linux and mac
    raise UnprocessableEntity, "unknown file type" if !mime_type
    mime_type.split(':')[1].split('/')[1].gsub('x-', '').gsub(/jpeg/, 'jpg').gsub(/text/, 'txt').gsub(/x-/, '')
  end  
end

Answer 4

The ruby-filemagic gem is good solution, but requires additional dependencies on libmagic (recently removed from CarrierWave as part of CarrierWave::MagicMimeTypes removal).

If you're interested in a pure ruby implementation, consider the MimeMagic gem! It works well for file types listed in the freedesktop.org mime database:

require 'mimemagic'

MimeMagic.by_magic(File.open('Table-Flip-Guy.jpg')).type # => "image/jpeg" 

For Microsoft Office 2007+ formats (xlsx, docx, and pptx), require the overlay (unless you're okay with the generic "application/zip" MIME type for these files)

require 'mimemagic'    
require 'mimemagic/overlay'

MimeMagic.by_magic(File.open('big_spreadsheet.xlsx')).type # => "application/vnd.openxmlformats-officedocument.spreadsheetml.sheet" 

Answer 5

filemagic gem is good solution but depends lots of unnecessary gems. (rails, aws-sdk-core, ...)

If your app is small and only runs in Linux or OSX, consider to use file program:

require 'shellwords'
mimetype = `file --brief --mime-type - < #{Shellwords.shellescape(__FILE__)}`.strip

Note: Replace __FILE__ with any expr contains the filepath.

Answer 6

mimemagic gem will also do it

https://github.com/minad/mimemagic

from the oficial documentation

MimeMagic is a library to detect the mime type of a file by extension or by content. It uses the mime database provided by freedesktop.org (see http://freedesktop.org/wiki/Software/shared-mime-info/).

require 'mimemagic'
MimeMagic.by_extension('html').text?
MimeMagic.by_extension('.html').child_of? 'text/plain'
MimeMagic.by_path('filename.txt')
MimeMagic.by_magic(File.open('test.html'))
# etc...

Answer 7

You can use

Mime::Type.lookup_by_extension(extention_name)

Thanks