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I wanted to add support for .wif files (text/plain) to my site (WP3.3.1). So I added the below function to my theme's functions.php. My theme is a child theme of Suffusion.

add_filter('upload_mimes', 'custom_upload_mimes');

function custom_upload_mimes ( $existing_mimes=array() ) {
  $existing_mimes['wif'] = 'text/plain';
  return $existing_mimes;

It had no effect and .wif files continued to generate the security message when I tried to upload them.

So I looked at the source code for get_allowed_mime_types(). The only entry for 'text/plain' is this 'txt|asc|c|cc|h' => 'text/plain'. Just for fun I edited it to say this: 'txt|asc|c|cc|h|wif' => 'text/plain'. That edit allowed .wif files to be uploaded.

But since editing core WP files is a bad idea, I tried another solution. On the assumption that perhaps values in the $mimes array had to be unique, I tried changing the key that points to 'text/plain' with this function:

add_filter('upload_mimes', 'custom_upload_mimes');

function custom_upload_mimes($mimes=array()){
    return $mimes;

This, however, also does not allow .wif file upload.

So I think that either the upload_mimes filter is not being applied for some reason. Or, my filter is being overwritten by another. I've tried both my functions with high (1) and low (PHP_MAX_INT) priority. It has no effect. I also checked for .htaccess directives and their weren't any. Any ideas?

ETA It turns out that some combination of plugins that includes BackUpWordPress causes filters attached to 'upload_mimes' to not run. I have not determined why this is and the people oat BackUpWordPress tell me that their plugin does not touch that filter.

share|improve this question

I would use:

add_filter( 'upload_mimes', 'theme_restrict_mime_types' );
function theme_restrict_mime_types( $mime_types )
    $mime_types = array(
        'wif' => 'text/plain',
        'doc|docx' => 'application/msword',
        'jpg|jpeg' => 'image/jpeg',
        'gif' => 'image/gif',
        'png' => 'image/png'
    return $mime_types;

In this example I list all types that I allow (with WIF included). So you would need to add whats missing for your liking.

This works on my WP 3.3.1 install.

share|improve this answer
It turns out that my filter is not the problem. The issue is a conflict with a backup plugin. When the plugin is off, my filter works fine. Thanks though. – dnagirl Apr 3 '12 at 18:40
this will kill all allowed mime types currently set by WP. Probably not a good idea. – chrisguitarguy Oct 13 '12 at 6:21
Maybe use $mime_types .= array(, so it appends them to the default ones. – Ciprian Nov 13 '12 at 9:19
@CiprianPopescu $mime_types += ... – kaiser Nov 13 '12 at 15:08
@kaiser += is for Javascript, .= is for PHP. – Ciprian Nov 14 '12 at 10:45

The problem is with the filter. Deregister the old key, to avoid conflicts. Then simply add your new one.

add_filter( 'upload_mimes', 'wpse44777_upload_mimes' );
function wpse44777_upload_mimes( $mime_types )
    // First we unregister the old key
    unset( $mime_types['txt|asc|c|cc|h'] );

    // Then we add a new one
    ! isset( $mime_types['txt|asc|c|cc|h|wif'] ) AND $mime_types['txt|asc|c|cc|h|wif'] = 'text/plain';

    return $mime_types;
share|improve this answer
Other than specifying the array index rather than searching for it, how is this different from my 2nd attempt (if($ek=array_search($v,$mimes)){...)? I unset the existing array member and then I add my own and return the array. And yet it only works if I turn off BackUpWordPress. – dnagirl Nov 13 '12 at 13:42
@dnagirl Have overseen your unset(). In this case, it's simple: Blame the plugin. And yes, yours isn't really different. Just a little less readable and slower :) Please add your solution (if you found out why BackUpWordPress was causing this issue) and maybe report that to the author. – kaiser Nov 13 '12 at 14:11
unfortunately, I haven't discovered why the plugin was causing the problem. Their codebase does not even touch the upload_mimes() filter. But if I deactivate it, all is good. BTW, the reason I used the loop was in case WP ever changed their key for 'text/plain'. – dnagirl Nov 13 '12 at 15:56

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