@hannes2peer @manuel The issue was that $config['attachments']['supported'] was set to "true" (instead of an array of mimetype=>extension bindings)

<MMN-o> The thing is that that array is used to get the file extension based on a mime type value
<MMN-o> and if it's set to true, we can't guess that
<MMN-o> so the solution would be to have a separate array for mime=>extension relations