The first message is a generic message produced by DNN when you try to upload files that aren't allowed by the Host.  The second is specific to the Reports module, because the module only allows HTML files to be uploaded.  I realize there appears to be some conflict in that the first says it accepts ZIP files and the second doesn't, but that's because the first message is the same for every file upload box in a DNN website (and some of those do allow ZIPs).

In order to prevent malicious users from being able to upload HTML files (and other files) with scripts, etc.  DNN only allows files with certain extensions to be uploaded.  The Host or "Super User" account can edit this list in the Host Settings page, but DNN does not allow Admins and Users to edit this list for security purposes.  If you are running your own portal, you can log in as the SuperUser and add "htm" and "html" to the list of extensions in the Host Setting page.  If not, you'll need to contact the person running the DotNetNuke application on which your portal is running.

Also, ZIPing the file won't get around the problem because DNN will only allow the zip to be uploaded if it only contains files with allowed extensions.

Andrew Nurse
DotNetNuke Core Team Member and Reports Module Project Lead
Microsoft Certified Professional Developer