Symptoms

Certain characters are unsupported in file names when using Attachment control but at the moment these characters are not listed explicitly in File Attachment Control documentation.
 

Diagnoses

There is a list of special characters not allowed in K2 Artifact names:
http://help.k2.com/KB000299
The same list is applicable to the SmartForms File Attachment Control. At the moment File Attachment control documentation does not include this list.
In K2 4.6.7/4.6.8 and SmartForms 1.0.6/1.0.7 you should also avoid using letters with diacritical symbols like umlauts, accents etc. which are used in certain languages (German, Icelandic etc.) in process names, forms names etc.
 

Resolution

Do not use special characters listed in KB000299 and letters with diacritical symbols for files and other objects names.