Web mail handles attachments differently from client applications. I personally use several web-based email accounts and not a single one gives you the view option. Although, the attachment is a graphics file, Yahoo gives you a preview of the attachment. Otherwise, the only option is to download it. Most services offer the option to scan the attachment then download or download without scan. If the file is an executable, you can "run from location" but this is inadvisable if you don't run full time virus scanning software.
That's probably something you did not want to hear but unfortunately that is the case with web based email. Client apps such as Outlook will allow you to view the attachment. If I understand you correctly, your company offers web access to email. Most mail servers that offer that option are also compatible with OUtlook, hence an exchange server. If the client app (i.e., Outlook) is installed, he should be able to view the attachment without downloading it.
Hope this helps and good luck!
Regards,
Jesse