AP-487: correctly declare mime type for mp3s

identified in code review of BerkeleyLibrary/avplayer#17. the declared
mime types as specified for FileType::MP3 erroneously are using the
mime type for m3u8 playlists instead of the correct mime type for
mp3 data.

Author: anarchivist

lib/berkeley_library/av/types/file_type.rb

@@ -56,7 +56,7 @@ def raise_if_duplicate(extensions)
         # ############################################################
         # Enum members
 
-        new(:MP3, 'audio', 'application/x-mpegURL', extensions: MP3_EXTENSIONS)
+        new(:MP3, 'audio', 'audio/mpeg', extensions: MP3_EXTENSIONS)
         new(:MP4, 'video', 'video/mp4', extensions: MP4_EXTENSIONS)
         new(:UNKNOWN, 'object', 'application/octet-stream', label: 'Unknown')
 

spec/lib/berkeley_library/av/types/file_type_spec.rb

@@ -7,7 +7,7 @@ module Types
         describe :mime_type do
           it 'is correct' do
             expected = {
-              FileType::MP3 => 'application/x-mpegURL',
+              FileType::MP3 => 'audio/mpeg',
               FileType::MP4 => 'video/mp4',
               FileType::UNKNOWN => 'application/octet-stream'
             }