Symptoms

On K2 SmartForms, when using the Attachment control.
The following message is raised when attempting to attach a control of a certain extension:
'The type of file may not be attached'
 

Diagnoses

Please have a look at the following URL:
http://help.k2.com/onlinehelp/k2smartforms/UserGuide/current/default.htm_SF-ViewControls-FileAttachmentWithProgress.html

Under the section: Attachment Control File Types and Size > Blacklist files

If the file type is listed in the list of Blacklisted files, the error will be expected
 

Resolution

In the web.config file of both the Runtime and Designer (depending on how the SmartForms are used):

* Searched for 'Blacklist
* Uncommented the line
* Searched for the requested filetype in the line of Blacklisted file types and removed it
* Saved the web.config and refreshed the browser