media assets: fix regenerating AI tags for flash files.

Fix it so that trying to regenerate AI tags for a Flash file doesn't
fail because Flash files have no image preview.

Also let `MediaFile.open` take a block argument.

app/logical/media_file.rb

@@ -15,17 +15,38 @@ class MediaFile
   # delegate all File methods to `file`.
   delegate *(File.instance_methods - MediaFile.instance_methods), to: :file
 
-  # Open a file or filename and return a MediaFile object.
+  # Open a file or filename and return a MediaFile object. If a block is given,
+  # pass the file to the block and return the result after closing the file.
   #
-  # @param file [File, String] a filename or an open File object
+  # @param file [File, MediaFile, String] A filename or an open File object.
   # @param options [Hash] extra options for the MediaFile subclass.
-  # @return [MediaFile] the media file
-  def self.open(file, **options)
-    return file.dup if file.is_a?(MediaFile)
+  # @yieldparam media_file [MediaFile] The opened media file.
+  # @return [MediaFile] The media file.
+  def self.open(file, **options, &block)
+    if file.is_a?(MediaFile)
+      media_file = file
+    else
+      file = Kernel.open(file, "r", binmode: true) unless file.respond_to?(:read)
+      media_file = new_from_file(file, **options)
+    end
 
-    file = Kernel.open(file, "r", binmode: true) unless file.respond_to?(:read)
+    if block_given?
+      result = yield media_file
+      media_file.close
+      result
+    else
+      media_file
+    end
+  end
 
-    case file_ext(file)
+  # Return a new MediaFile from an open File object.
+  #
+  # @param file [File] The File object.
+  # @param file_ext [Symbol] The file extension.
+  # @param options [Hash] Extra options for the MediaFile subclass.
+  # @return [MediaFile] The media file.
+  def self.new_from_file(file, file_ext = MediaFile.file_ext(file), **options)
+    case file_ext
     when :jpg, :gif, :png, :webp, :avif
       MediaFile::Image.new(file, **options)
     when :swf