null in multi-part parameter value

Dear All,

My customer wants a restriction on a parameter type in the url. The customer wants to allow only doc/docx/pdf files for the url. I did the below steps to create the parameter. 

Create the parameter for the security policy

i. Parameter Level: Global ii. Parameter Value Type: User-Input iii. Data Type >Check Max Length: Give a number (e.g. 100) iv. Data Type >Regular expression: (?i).( pdf|doc|docx) Note:- (?i) – ignore case sensitivity

When I apply it to the Security policy & try to upload the docx file, the docx file is blocked. The log says the below error "null in multi-part parameter value". The customer is using 12.1.2 version

I could see some old discussion on this in the below link, but there is no solution mentioned

https://devcentral.f5.com/questions/not-able-to-attach-doc-excel-in-f5-ver-113-asm-blocking-mode

Any help is appreciated

Regards Nikhil