Trying to upload an attachment to phpBB from some mobile devices running Android 4.x results in the following error:
The image file you tried to attach is invalid.
This is caused by an invalid mimetype, returned by the default Android browser. The incorrect mimetype is: application/octet-stream
phpBB looks if it contains image/ and is therefore failing the check and rejecting the attachment.
I've come to this conclusion by upload tests from people on different mobile devices, operating systems and browsers:
(Thanks for testing everyone)
Please note that disabling the ACP setting "Check attachment files, Some browsers can be tricked to assume an incorrect mimetype for uploaded files.." will not help because it's checked in includes/functions_posting.php, regardless of the setting. (the setting is only working in includes/functions_upload.php which is runned after functions_posting)