setting content-type for mp4 files on s3 - ruby-on-rails

Question

I am adding user uploaded videos to my RoRs site with the help of the paperclip gem and s3 storage. For some reason that I can't figure out, whenever a user uploads an mp4 file, s3 sets content-type for that file as application/mp4 instead of video/mp4.
Note that I have registered mp4 mime type in an initializer file:
Mime::Type.lookup_by_extension('mp4').to_s
=> "video/mp4"
Here is the relevant part of my Post model:
has_attached_file :video,
:storage => :s3,
:s3_credentials => "#{Rails.root.to_s}/config/s3.yml",
:path => "/video/:id/:filename"
validates_attachment_content_type :video,
:content_type => ['video/mp4'],
:message => "Sorry, this site currently only supports MP4 video"
What am I missing in my paperclip and/or s3 set-up.
####Update#####
For some reason that is beyond my knowledge of Rails, my default mime types for mp4 contained files is the following:
MIME::Types.type_for("my_video.mp4").to_s
=> "[application/mp4, audio/mp4, video/mp4, video/vnd.objectvideo]"
So, when paperclip send an mp4 file to s3, it seems to identify the file's mime type as the first default, "application/mp4". That is why s3 identifies the file as having a content-type of "application/mp4". Because I want to enable streaming of these mp4 files, I need paperclip to identify the file as having a mime type of "video/mp4".
Is there a way to modify paperclip (maybe in a before_post_process filter) to allow for this, or is there a way to modify rails through an init file to identify mp4 files as being "video/mp4". If I could do either, which way is best.
Thanks for your help

Answer 1

Turns out that I needed to set a default s3 header content_type in the model. This isn't the best solution for me because at some point I might start allowing video containers other than mp4. But it gets me moving on to the next problem.
has_attached_file :video,
:storage => :s3,
:s3_credentials => "#{Rails.root.to_s}/config/s3.yml",
:path => "/video/:id/:filename",
:s3_headers => { "Content-Type" => "video/mp4" }

Answer 2

I did the following:
...
MIN_VIDEO_SIZE = 0.megabytes
MAX_VIDEO_SIZE = 2048.megabytes
VALID_VIDEO_CONTENT_TYPES = ["video/mp4", /\Avideo/] # Note: The regular expression /\Avideo/ will match anything that starts with "video"
has_attached_file :video, {
url: BASE_URL,
path: "video/:id_partition/:filename"
}
validates_attachment :video,
size: { in: MIN_VIDEO_SIZE..MAX_VIDEO_SIZE },
content_type: { content_type: VALID_VIDEO_CONTENT_TYPES }
before_validation :validate_video_content_type, on: :create
before_post_process :validate_video_content_type
def validate_video_content_type
if video_content_type == "application/octet-stream"
# Finds the first match and returns it.
# Alternatively you could use the ".select" method instead which would find all mime types that match any of the VALID_VIDEO_CONTENT_TYPES
mime_type = MIME::Types.type_for(video_file_name).find do |type|
type.to_s.match Regexp.union(VALID_VIDEO_CONTENT_TYPES)
end
self.video_content_type = mime_type.to_s unless mime_type.blank?
end
end
...