How-To Create/Edit Local Config Files (.local.php)

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First read Configuration Files

Applying changes to the files in config/ (does not apply to conf.php) as this is managed though the web-interface and updated automatically) directories is not recommended as when you update Horde modules these files will be replaced.
Horde has supplied us a method to get around this problem with local overrides which are not overwritten each time the modules are updated. There are also other methods such as vhosts or prefs.d/ which this article does not go into directly but the principal is the same.

There are a few ways to add entries into the local files.

• Replace full array

$mime_drivers = array(
    /* HTML driver settings */
    'html' => array(
        /* NOTE: Inline HTML display is turned OFF by default. */
        'inline' => false,
        'handles' => array(
            'text/html'
        ),
        'icons' => array(
            'default' => 'html.png'
        ),

        /* If you want to limit the display of message data inline for large
         * messages, set the maximum size of the displayed message here (in
         * bytes).  If exceeded, the user will only be able to download the
         * part. Set to 0 to disable this check. */
        'limit_inline_size' => 1048576,

        /* Check for phishing exploits? */
        'phishing_check' => true
    )
);

• Replace individual array entry

/* HTML driver settings */
$mime_drivers['html'] = array(
        /* NOTE: Inline HTML display is turned OFF by default. */
        'inline' => false,
        'handles' => array(
            'text/html'
        ),
        'icons' => array(
            'default' => 'html.png'
        ),

        /* If you want to limit the display of message data inline for large
         * messages, set the maximum size of the displayed message here (in
         * bytes).  If exceeded, the user will only be able to download the
         * part. Set to 0 to disable this check. */
        'limit_inline_size' => 1048576,

        /* Check for phishing exploits? */
        'phishing_check' => true
    );

• Modify individual entries within the array

$mime_drivers['html']['inline']=true;

• Merge array to existing array (useful in the case of the format seen in mime_drivers.php)

$mime_drivers=array_merge($mime_drivers,array(
    /* Plain text viewer. */
    'plain' => array(
        'inline' => true,
        'handles' => array(
            'application/pgp',
            'text/plain',
            'text/rfc822-headers'
        ),

        /* If you want to limit the display of message data inline for large
         * messages, set the maximum size of the displayed message here (in
         * bytes).  If exceeded, the user will only be able to download the
         * part. Set to 0 to disable this check. */
        'limit_inline_size' => 1048576,

        /* If you want to scan ALL incoming text/plain messages for UUencoded
         * data, set the following to true. This is very performance intensive
         * and can take a long time for large messages. It is not recommended
         * (as UUencoded data is rare these days) and is disabled by
         * default. */
        'uudecode' => false
    ),

    /* HTML driver settings */
    'html' => array(
        /* NOTE: Inline HTML display is turned OFF by default. */
        'inline' => false,
        'handles' => array(
            'text/html'
        ),
        'icons' => array(
            'default' => 'html.png'
        ),

        /* If you want to limit the display of message data inline for large
         * messages, set the maximum size of the displayed message here (in
         * bytes).  If exceeded, the user will only be able to download the
         * part. Set to 0 to disable this check. */
        'limit_inline_size' => 1048576,

        /* Check for phishing exploits? */
        'phishing_check' => true
    )
));