Hi,

Here are some things to check with regards to uploading/downloading attachments:-

1. Is the Form that contains the attachment control being configured for Anonymous Access? If so, you need to make some configuration changes that will allow users to upload and download files using an Anonymous Access Form. More details can be found here:-

http://help.k2.com/onlinehelp/k2smartforms/userguide/4.7/default.htm#sf-viewcontrols-fileattachmentwithprogress.html

2. From what I know on uploading attachments on an iOS device, Forms authentication must be enabled (in turn Windows authentication disabled) on the SmartForms runtime app in IIS to upload files. Otherwise, I have seen a case where the control is stuck at 'Uploading' and the attachment never gets uploaded. 

As for the download issue on an Android app, are there any errors encountered during the process? More information or screenshots provided will be helpful here.