How to create thumbnails only for image files with Paperclip? - ruby-on-rails

Question

I use the following code to create Assets from the uploaded files:
def upload
uploader = User.find_by_id(params[:uploader_id])
params[:assets].each do |file|
new_asset = uploader.assets.build(:asset => file) # Here the error appears
new_asset.save
end
...
end
I noticed that when I upload non-image files, e.g. my.xlsx, I got the following error:
[paperclip] identify -format %wx%h "C:/temp/stream20110628-460-3vqjnd.xlsx[0]" 2>NUL
[paperclip] An error was received while processing:
#<Paperclip::NotIdentifiedByImageMagickError: C:/temp/stream20110628-460-3vqjnd.xlsx is
not recognized by the 'identify' command.>
(For image files everything works fine: a thumbnail is created, and there is no error.)
Is that because Paperclip tries to create a thumbnail from my.xlsx ?
What configuration will create thumbnails only for image files ?
Here is some relevant code:
class Asset < ActiveRecord::Base
belongs_to :uploader, :class_name => "User"
has_attached_file :asset, :styles => { :thumb => "80x80#" }
end

Answer 1

I used the following nice solution:
before_post_process :image?
def image?
(asset_content_type =~ SUPPORTED_IMAGES_REGEX).present?
end
where:
SUPPORTED_IMAGE_FORMATS = ["image/jpeg", "image/png", "image/gif", "image/bmp"]
SUPPORTED_IMAGES_REGEX = Regexp.new('\A(' + SUPPORTED_IMAGE_FORMATS.join('|') + ')\Z')

Answer 2

Change the has_attached_file line to read:
has_attached_file :asset, :styles => { :thumb=> "80x80#" }, :whiny_thumbnails => false
This will prevent it from raising an error when thumbnails are not created. Note though that it won't raise errors if one occurs when processing an image though.