MTM Screenshot image url problem, access from outside of corpnet

Hi Geniuses,

We got problem when accessing screenshot captured in MTM in work items - repro steps section from outside of corp net,

I checked the URL of the screenshot, it still points to the host name (tfs) or the internal FQDN (tfs.corp.local),

but from outside, people are using tfs.companyname.com and the dns and port mapping are all good,

so probably, my question is, how do I let TFS know, my public DNS name is tfs.companyname.com?

btw, I already set the IIS TFS site bindings to the tfs.companyname.com.

oh, I would say everything works fine from outside except the screenshots :(

Help! Need help!

Looking for your suggestions, any answer will be appreciated!

TFS build version 12.0.30723.0 (Tfs2013.Update3)