I am finding the below error when adding attachmnets in web client. Please see the attacghed screenshot
Is it happening on any file or only on this exact one?
I think you may have the following known issue:
http://support.openview.hp.com/selfsolve/document/KM00763941Attachment can not be saved if the parameter attachmentsegmentsize has a large value
You can also try with a web.xml of OOB system.
You can apply it and see if it is possible to do the attach of the file.