media file: factor out file type detection code.

Factor out the file type detection code from MediaFile into a FileTypeDetector class so we can use
it to detect archive files (.zip, .rar, .7z) too.

app/logical/danbooru/archive.rb

@@ -155,7 +155,12 @@ module Danbooru
       false
     end
 
-    # @return [String] The archive format ("RAR", "ZIP", etc).
+    # @return [Symbol] The archive format as detected by us (:zip, :rar, :7z, etc).
+    def file_ext
+      @file_ext ||= FileTypeDetector.new(file).file_ext
+    end
+
+    # @return [String] The archive format as returned by libarchive ("RAR", "ZIP", etc).
     def format
       @format ||= entries.lazy.map(&:format).first
     end

app/logical/file_type_detector.rb

@@ -0,0 +1,71 @@
+# frozen_string_literal: true
+
+# Detect a file's type based on its file signature.
+#
+# @see https://en.wikipedia.org/wiki/List_of_file_signatures
+class FileTypeDetector
+  extend Memoist
+  attr_reader :file
+
+  # @param [File] The file to detect.
+  def initialize(file)
+    @file = file
+  end
+
+  # @return [Symbol] The file's extension (e.g. :jpg, :png, etc). Returns `:bin` if the file type is unknown.
+  memoize def file_ext
+    header = file.pread(16, 0)
+
+    case header
+    when /\A\xff\xd8/n
+      :jpg
+    when /\AGIF87a/, /\AGIF89a/
+      :gif
+    when /\A\x89PNG\r\n\x1a\n/n
+      :png
+    when /\ACWS/, /\AFWS/, /\AZWS/
+      :swf
+
+    # This detects the Matroska (.mkv) header. WebM files have a DocType of "webm", which is checked later in `MediaFile::Video#is_supported?`.
+    #
+    # https://www.rfc-editor.org/rfc/rfc8794.html#section-8.1
+    # https://www.webmproject.org/docs/container/
+    when /\A\x1a\x45\xdf\xa3/n
+      :webm
+
+    # https://developers.google.com/speed/webp/docs/riff_container
+    when /\ARIFF....WEBP/
+      :webp
+
+    # https://www.ftyps.com
+    # https://cconcolato.github.io/mp4ra/filetype.html
+    # https://github.com/mozilla/gecko-dev/blob/master/toolkit/components/mediasniffer/nsMediaSniffer.cpp#L78
+    # https://mimesniff.spec.whatwg.org/#signature-for-mp4
+    #
+    # isom (common) - MP4 Base Media v1 [IS0 14496-12:2003]
+    # mp42 (common) - MP4 v2 [ISO 14496-14]
+    # iso4 (rare) - MP4 Base Media v4
+    # iso5 (rare) - MP4 Base Media v5 (used by Twitter)
+    # 3gp5 (rare) - 3GPP Media (.3GP) Release 5 (XXX technically this should be .3gp, not .mp4. Supported by Chrome but not Firefox)
+    # avc1 (rare) - MP4 Base w/ AVC ext [ISO 14496-12:2005]
+    # M4V (rare) - Apple iTunes Video (https://en.wikipedia.org/wiki/M4V)
+    when /\A....ftyp(?:mp4|avc|iso|3gp5|M4V)/
+      :mp4
+
+    # https://aomediacodec.github.io/av1-avif/#brands-overview
+    when /\A....ftyp(?:avif|avis)/
+      :avif
+    when /\APK\x03\x04/
+      :zip
+    else
+      :bin
+    end
+  rescue EOFError
+    :bin
+  end
+
+  # @return [String] The file's MIME type, or "application/octet-stream" if unknown.
+  def mime_type
+    Mime::Type.lookup_by_extension(file_ext).to_s.presence || "application/octet-stream"
+  end
+end

app/logical/media_file.rb

@@ -64,54 +64,7 @@ class MediaFile
   # @param [File] an open file
   # @return [Symbol] the file's type
   def self.file_ext(file)
-    header = file.pread(16, 0)
-
-    case header
-    when /\A\xff\xd8/n
-      :jpg
-    when /\AGIF87a/, /\AGIF89a/
-      :gif
-    when /\A\x89PNG\r\n\x1a\n/n
-      :png
-    when /\ACWS/, /\AFWS/, /\AZWS/
-      :swf
-
-    # This detects the Matroska (.mkv) header. WebM files have a DocType of "webm", which is checked later in `MediaFile::Video#is_supported?`.
-    #
-    # https://www.rfc-editor.org/rfc/rfc8794.html#section-8.1
-    # https://www.webmproject.org/docs/container/
-    when /\A\x1a\x45\xdf\xa3/n
-      :webm
-
-    # https://developers.google.com/speed/webp/docs/riff_container
-    when /\ARIFF....WEBP/
-      :webp
-
-    # https://www.ftyps.com
-    # https://cconcolato.github.io/mp4ra/filetype.html
-    # https://github.com/mozilla/gecko-dev/blob/master/toolkit/components/mediasniffer/nsMediaSniffer.cpp#L78
-    # https://mimesniff.spec.whatwg.org/#signature-for-mp4
-    #
-    # isom (common) - MP4 Base Media v1 [IS0 14496-12:2003]
-    # mp42 (common) - MP4 v2 [ISO 14496-14]
-    # iso4 (rare) - MP4 Base Media v4
-    # iso5 (rare) - MP4 Base Media v5 (used by Twitter)
-    # 3gp5 (rare) - 3GPP Media (.3GP) Release 5 (XXX technically this should be .3gp, not .mp4. Supported by Chrome but not Firefox)
-    # avc1 (rare) - MP4 Base w/ AVC ext [ISO 14496-12:2005]
-    # M4V (rare) - Apple iTunes Video (https://en.wikipedia.org/wiki/M4V)
-    when /\A....ftyp(?:mp4|avc|iso|3gp5|M4V)/
-      :mp4
-
-    # https://aomediacodec.github.io/av1-avif/#brands-overview
-    when /\A....ftyp(?:avif|avis)/
-      :avif
-    when /\APK\x03\x04/
-      :zip
-    else
-      :bin
-    end
-  rescue EOFError
-    :bin
+    FileTypeDetector.new(file).file_ext
   end
 
   # @return [Boolean] true if we can generate video previews.