Allow to resolve known mime type to file extension

[mvorisek]

Description

Mime type to file extension is already supported by FILEINFO_EXTENSION but it is impossible to specify the mime type :).

I expect code like:

$mimetype = 'application/zip';
$finfo = finfo_open(FILEINFO_EXTENSION);
var_dump(finfo_buffer($finfo, $mimetype, FILEINFO_EXTENSION)); // notice $mimetype is mime type not file data

to dump zip.

It should be possible to find the file magic data by iterating the magic data like in https://github.com/file/file/blob/FILE5_45/src/softmagic.c#L135, the mime/ext is defined in https://github.com/file/file/blob/FILE5_45/src/file.h#L371.

[mvorisek]

May I know if there is currently any workaround? I would be happy if https://github.com/roundcube/roundcubemail/blob/1.6.5/program/lib/Roundcube/rcube_mime.php#L835 can be replaced /wo having to bundle mime.types, as the file is large and in fileinfo ext it is bundled already: https://github.com/php/php-src/blob/master/ext/fileinfo/data_file.c

[nielsdos]

May I know if there is currently any workaround?

I'm pretty sure the necessary APIs to work around this aren't exposed to userland currently.

[nielsdos]

I looked into this again. Libmagic does not provide an API to do this directly, it only works with input bytes. This would require us to loop over the struct magic entries, match mimetype and then get the matching extensions. The way the data is structured in memory is also not really ideal for this kind of lookup.