Seems there is a mismatch between the Frontpage extensions on workstations and those on our server. Microsoft is aware of the issue, but with the announcement of discontinuing Frontpage in the near future, the problem is probably never going to be resolved.
The immediate work around for this problem is for users to upload only content and not their extensions when working in Frontpage. An alternative also to download a copy of Microsoft Sharepoint Designer 2007 , Visual Studio Express, Expression Web or Visual Web Developer. All these applications are geared to the hobbiest and small business. Some are betas and others are for purchase or free. Links to these products are listed below.
~Your Support Team
SharePoint Designer Download:
http://www.microsoft.com/downloads/details.aspx?FamilyId=BAA3AD86-BFC1-4BD4-9812-D9E710D44F42&displaylang=en
Visual Studio Express:
http://msdn.microsoft.com/vstudio/express/
Visual Web Developer:
http://asp.net/downloads/essential.aspx?tabid=62
Expression Web:
http://www.microsoft.com/products/expression/en/default.mspx