I am going nuts

Trying to send attached files in Outlook 2010. The system will allow me to send PDF files (any size) but I can't send f.x. a JPG file (even when very small) or a DOC file.

I have manage to eliminate that the fault is caused by either a firewall, a mailguard or virus protection. This is definitely not the problem.

I have therefore come to the conclusion that the fault must be a setting in Outlook 2010 but I can't work out where to make changes.

Any help is appreciated.