HTTP content type video / mp4 warning in firefox

I have a video that is embedded in a web page.

My htaccess:

<IfModule mod_rewrite.c>
    RewriteEngine On
    RewriteCond %{REQUEST_FILENAME} !-f
    RewriteRule ^(.*)$ app.php [QSA,L]
    AddType video/ogg .ogv
    AddType video/mp4 .mp4
    AddType video/webm .webm
</IfModule>

HTML:

<video autoplay="false" width="586px" height="440" src="...mp4"></video>

I am using mediaelement.js player:

$('video').mediaelementplayer();

In firefox, the video does not load and the preview image is not displayed. The player is just black. In firebug, I get this warning:

HTTP "Content-Type" "video / mp4" is not supported. Failed to load media ...

Any idea how to solve this problem?