Sometimes you don't want image upload in the model. Here's a controller class I use to upload a recipe image, standardize the size, create a thumbnail, and write them out.

This code uses a "attr_accessor :file" in the recipe model. file is not a table field, it's just a temporary variable for storing the data, before I load it into RMagick.

def do_upload_image
         image_types = ["image/jpeg", "image/pjpeg", "image/gif", "image/png", "image/x-png"]
         
         @recipe = Recipe.find(params[:id])
         @recipe.update_attributes(params[:recipe])
         logger.info "@recipe.file = " + @recipe.file.to_s
         if !@recipe.file.blank?
                 if (@recipe.file.size != 0) 
                         logger.info "content_type = " + @recipe.file.content_type.to_s
                         logger.info "size = " + @recipe.file.size.to_s
                         if (image_types.include?@recipe.file.content_type.chomp)
                         
                                 if @recipe.file.size < 2097152
                                         @recipe.file.rewind
                                         
                                         # Load the file field directly into the blob, straight from memory and into RMagick!  No need to write the raw file out to the filesystem
                                         pic = Magick::Image.from_blob(@recipe.file.read)[0]

                                         # I want to place the image inside a 200x200 jpg, with a white background. 
                                         # so all my images are the same size, while maintaning proportions.
                                         width = pic.columns
                                         height = pic.rows
                                         if (width > height)
                                                 pic.scale!((200.0/width.to_f))
                                         else
                                                 pic.scale!((200.0/height.to_f))
                                         end
                                         
                                         back = Magick::Image.new(200,200) {
                                                 self.background_color = 'white'
                                                 self.format = 'JPG'
                                         }
                                         back.composite!(pic, Magick::CenterGravity, Magick::InCompositeOp)
                                         
                                         # Now scale this down for a thumbnail
                                         thumb = back.scale(50,50)
                                         
                                         # Add a rand to the filename, so if we upload a new image, we're shown it, instead of whatever we have cached.
                                         rand = Kernel.rand(1000).to_s

                                         File.open(RAILS_ROOT + "/public/images/recipes/" + @recipe.id.to_s + "-" + rand + ".jpg", "wb") do |f|
                                                 f.write(back.to_blob)
                                         end
                                         File.open(RAILS_ROOT + "/public/images/recipes/" + @recipe.id.to_s + "-" + rand + "t.jpg", "wb") do |f|
                                                 f.write(thumb.to_blob)
                                         end

                                         @recipe.picture = "/images/recipes/" + @recipe.id.to_s + "-" + rand + "." + "jpg"
                                         @recipe.thumbnail = "/images/recipes/" + @recipe.id.to_s + "-" + rand + "t." + "jpg"
                                         @recipe.file = nil
                                         @recipe.save
                                         
                                         flash[:notice] = 'Image Uploaded and Thumbnailed'
                                         redirect_to :action => "edit", :id => @recipe
                                         #session[:recipe] = @recipe
                                 else
                                         flash.now[:error] = "File size greater then 2 Megabytes, please upload a smaller file."    
                                         render :action => 'upload_image', :id => @recipe
                                 end 
                         else 
                                 flash.now[:error] = "File doesn't seem to be JPG, GIF, or PNG, please ensure it is a valid image file."    
                                 render :action => 'upload_image', :id => @recipe
                         end
                 else
                         flash.now[:error] = "Unable to upload the file you selected, please try again."    
                         render :action => 'upload_image', :id => @recipe
                 end
         else
                 flash.now[:error] = "Unable to upload the file you selected, please try again."    
                 render :action => 'upload_image', :id => @recipe
         end
         
 end