Ruby on Rails: How do you check if a file is an image?

Question

How would you check if a file is an image? I\'m thinking you could use an method like so:

def image?(file)
  file.to_s.include?(\".gif\") or file.to_s.includ         


        

Answer 1

Please check it once

MIME::Types.type_for('tmp/img1.jpg').first.try(:media_type)
=> "image"

MIME::Types.type_for('tmp/img1.jpeg').first.try(:media_type)
=> "image"

MIME::Types.type_for('tmp/img1.gif').first.try(:media_type)
=> "image"

MIME::Types.type_for('tmp/ima1.png').first.try(:media_type)
=> "image"

Answer 2

Since you're using Paperclip, you can use the built in "validates_attachment_content_type" method in the model where "has_attached_file" is used, and specify which file types you want to allow.

Here's an example from an application where users upload an avatar for their profile:

has_attached_file :avatar, 
                  :styles => { :thumb => "48x48#" },
                  :default_url => "/images/avatars/missing_avatar.png",
                  :default_style => :thumb

validates_attachment_content_type :avatar, :content_type => ["image/jpeg", "image/pjpeg", "image/png", "image/x-png", "image/gif"]

The documentation is here http://dev.thoughtbot.com/paperclip/classes/Paperclip/ClassMethods.html

Answer 3

One approach is to use the "magic number" convention to read the first bits of a file.
http://www.astro.keele.ac.uk/oldusers/rno/Computing/File_magic.html

Examples:

  "BM" is a Bitmap image
  "GIF8" is a GIF image
  "\xff\xd8\xff\xe0" is a JPEG image

Example in Ruby:

  def bitmap?(data)
    return data[0,2]=="MB"
  end

  def gif?(data)
    return data[0,4]=="GIF8"
  end

  def jpeg?(data)
    return data[0,4]=="\xff\xd8\xff\xe0"
  end

  def file_is_image?(filename)
    f = File.open(filename,'rb')  # rb means to read using binary
    data = f.read(9)              # magic numbers are up to 9 bytes
    f.close
    return bitmap?(data) or gif?(data) or jpeg?(data)
  end

Why use this instead of the file name extension or the filemagic module?

To detect the data type before writing any data to disk. For example, we can read upload data stream before we write any data to disk. If the magic number doesn't match the web form content type, then we can immediately report an error.

We implement our real-world code slightly differently. We create a hash: each key is a magic number string, each value is a symbol like :bitmap, :gif, :jpeg, etc. If anyone would like to see our real-world code, feel free to contact me here.

Answer 4

imagemagick has a command called identity that handles this - check w/ the paperclip documentation - there's probably a way to handle this from within your RoR app.

Answer 5

As an addition to Joel's answer, in Rails 5 I had to transform the comparison string to a bytecode. Eg:

def jpeg?(data)
  return data[0,4]=="\xff\xd8\xff\xe0".b
end

Answer 6

I would use the ruby-filemagic gem which is a Ruby binding for libmagic.