Okay, please open application/config/mimes.php and add types like below, I think you are using CI < 3.0.0 that is why mp4 type is missing in mimes.php
'mp4' => 'video/mp4',
...
...
Here is a content of mimes.php
[akshay@localhost config]$ cat mimes.php
<?php if ( ! defined('BASEPATH')) exit('No direct script access allowed');
/*
| -------------------------------------------------------------------
| MIME TYPES
| -------------------------------------------------------------------
| This file contains an array of mime types. It is used by the
| Upload class to help identify allowed file types.
|
*/
return array('hqx' => array('application/mac-binhex40', 'application/mac-binhex', 'application/x-binhex40', 'application/x-mac-binhex40'),
'cpt' => 'application/mac-compactpro',
'csv' => array('text/x-comma-separated-values', 'text/comma-separated-values', 'application/octet-stream', 'application/vnd.ms-excel', 'application/x-csv', 'text/x-csv', 'text/csv', 'text/plain', 'application/csv', 'application/excel', 'application/vnd.msexcel', 'text/x-c'),
'bin' => array('application/macbinary', 'application/mac-binary', 'application/octet-stream', 'application/x-binary', 'application/x-macbinary'),
'dms' => 'application/octet-stream',
'lha' => 'application/octet-stream',
'lzh' => 'application/octet-stream',
'exe' => array('application/octet-stream', 'application/x-msdownload'),
'class' => 'application/octet-stream',
'psd' => 'application/x-photoshop',
'so' => 'application/octet-stream',
'sea' => 'application/octet-stream',
'dll' => 'application/octet-stream',
'oda' => 'application/oda',
'pdf' => array('application/pdf', 'application/x-pdf', 'application/x-download','application/x-download', 'binary/octet-stream', 'application/unknown', 'application/force-download'),
'ai' => 'application/postscript',
'eps' => 'application/postscript',
'ps' => 'application/postscript',
'smi' => 'application/smil',
'smil' => 'application/smil',
'mif' => 'application/vnd.mif',
'xls' => array('application/excel', 'application/vnd.ms-excel', 'application/msexcel', 'application/vnd.ms-office'),
'ppt' => array('application/powerpoint', 'application/vnd.ms-powerpoint', 'application/vnd.ms-office'),
'pptx' => array('application/vnd.openxmlformats-officedocument.presentationml.presentation', 'application/zip'),
'wbxml' => 'application/wbxml',
'wmlc' => 'application/wmlc',
'dcr' => 'application/x-director',
'dir' => 'application/x-director',
'dxr' => 'application/x-director',
'dvi' => 'application/x-dvi',
'gtar' => 'application/x-gtar',
'gz' => 'application/x-gzip',
'gzip' => 'application/x-gzip',
'php' => 'application/x-httpd-php',
'php4' => 'application/x-httpd-php',
'php3' => 'application/x-httpd-php',
'phtml' => 'application/x-httpd-php',
'phps' => 'application/x-httpd-php-source',
'js' => 'application/x-javascript',
'swf' => 'application/x-shockwave-flash',
'sit' => 'application/x-stuffit',
'tar' => 'application/x-tar',
'tgz' => array('application/x-tar', 'application/x-gzip-compressed'),
'xhtml' => 'application/xhtml+xml',
'xht' => 'application/xhtml+xml',
'zip' => array('application/x-zip', 'application/zip', 'application/x-zip-compressed'),
'mid' => 'audio/midi',
'midi' => 'audio/midi',
'mpga' => 'audio/mpeg',
'mp2' => 'audio/mpeg',
'mp3' => array('audio/mpeg', 'audio/mpg', 'audio/mpeg3', 'audio/mp3'),
'aif' => array('audio/x-aiff', 'audio/aiff'),
'aiff' => array('audio/x-aiff', 'audio/aiff'),
'aifc' => 'audio/x-aiff',
'ram' => 'audio/x-pn-realaudio',
'rm' => 'audio/x-pn-realaudio',
'rpm' => 'audio/x-pn-realaudio-plugin',
'ra' => 'audio/x-realaudio',
'rv' => 'video/vnd.rn-realvideo',
'wav' => 'audio/x-wav',
'bmp' => array('image/bmp', 'image/x-windows-bmp'),
'gif' => 'image/gif',
'jpeg' => array('image/jpeg', 'image/pjpeg'),
'jpg' => array('image/jpeg', 'image/pjpeg'),
'jpe' => array('image/jpeg', 'image/pjpeg'),
'png' => array('image/png', 'image/x-png'),
'tiff' => 'image/tiff',
'tif' => 'image/tiff',
'css' => 'text/css',
'html' => 'text/html',
'htm' => 'text/html',
'shtml' => 'text/html',
'txt' => 'text/plain',
'text' => 'text/plain',
'log' => array('text/plain', 'text/x-log'),
'rtx' => 'text/richtext',
'rtf' => 'text/rtf',
'xml' => array('application/xml', 'text/xml'),
'xsl' => 'text/xml',
'mpeg' => 'video/mpeg',
'mpg' => 'video/mpeg',
'mpe' => 'video/mpeg',
'qt' => 'video/quicktime',
'mov' => 'video/quicktime',
'avi' => array('video/x-msvideo', 'video/msvideo', 'video/avi', 'application/x-troff-msvideo'),
'movie' => 'video/x-sgi-movie',
'doc' => 'application/msword',
'docx' => array('application/vnd.openxmlformats-officedocument.wordprocessingml.document', 'application/zip'),
'odt' => array('application/vnd.oasis.opendocument.text'),
'xlsx' => array('application/vnd.openxmlformats-officedocument.spreadsheetml.sheet', 'application/zip'),
'word' => array('application/msword', 'application/octet-stream'),
'xl' => 'application/excel',
'eml' => 'message/rfc822',
'json' => array('application/json', 'text/json'),
'pem' => array('application/x-x509-user-cert', 'application/x-pem-file', 'application/octet-stream'),
'p10' => array('application/x-pkcs10', 'application/pkcs10'),
'p12' => 'application/x-pkcs12',
'p7a' => 'application/x-pkcs7-signature',
'p7c' => array('application/pkcs7-mime', 'application/x-pkcs7-mime'),
'p7m' => array('application/pkcs7-mime', 'application/x-pkcs7-mime'),
'p7r' => 'application/x-pkcs7-certreqresp',
'p7s' => 'application/pkcs7-signature',
'crt' => array('application/x-x509-ca-cert', 'application/x-x509-user-cert', 'application/pkix-cert'),
'crl' => array('application/pkix-crl', 'application/pkcs-crl'),
'der' => 'application/x-x509-ca-cert',
'kdb' => 'application/octet-stream',
'pgp' => 'application/pgp',
'gpg' => 'application/gpg-keys',
'sst' => 'application/octet-stream',
'csr' => 'application/octet-stream',
'rsa' => 'application/x-pkcs7',
'cer' => array('application/pkix-cert', 'application/x-x509-ca-cert'),
'3g2' => 'video/3gpp2',
'3gp' => 'video/3gp',
'mp4' => 'video/mp4',
'm4a' => 'audio/x-m4a',
'f4v' => 'video/mp4',
'aac' => 'audio/x-acc',
'm4u' => 'application/vnd.mpegurl',
'm3u' => 'text/plain',
'xspf' => 'application/xspf+xml',
'vlc' => 'application/videolan',
'wmv' => 'video/x-ms-wmv',
'au' => 'audio/x-au',
'ac3' => 'audio/ac3',
'flac' => 'audio/x-flac',
'ogg' => 'audio/ogg',
'pages' => 'application/zip',
'numbers' => 'application/zip',
'svg' => 'image/svg+xml',
'webm' => 'video/webm',
'ogg' => 'video/ogv'
);
/* End of file mimes.php */