A MIME-TYPE is a server side setting. You can ask your hosting provider to add the MP4 MIME-TYPE for you.
Or, you can add a .htaccess file to the directory where you uploads are stored.
A sample .htaccess file that adds the MP4 MIME-TYPE is:
<IfModule mod_rewrite.c>
AddType video/mp4 .mp4
</IfModule>
If you are not able to add the MIME TYPE properly, one work around you can use is to force the JW Player to run as Flash, which is less strict about MIME-TYPEs, by adding this line, to your player's embed code:
primary: 'flash',
Let me know if this helps!