Warning message when exporting a Financial Report to Excel 2007

Question

Excel 2007 users that attempt to export a fully formatted Financial Report into Excel 2007 get the following error message:
"The file you are trying to open, 'FileName.xls', is in a different format than specified by the file extension. Verify that the file is not corrupted and is from a trusted source before opening the file. Do you want to open the file now?"
I am still researching the behavior that users get when they click through the message, but would ideally like for the message not to appear without updating the client machine registries. Has anybody else run across this?
Excel 2007
BI Plus version: 9.3.1.0.0.248

Answer 1

It looks like this is a known issue addressed in Bug 7359171 - EXPORT TO EXCEL 2007 CAUSES FILE FORMAT WARNING:
The current design does not allow you to open HTML content from a web site in Excel unless the extension of the URL is .HTM/.HTML/.MHT/.MHTML. So ASP pages that return HTML and set the MIME type to something like XLS to try to force the HTML to open in Excel instead of the web browser (as expected) will always get the security alert since the content does not match the MIME type. If you use an HTML MIME type, then the web browser will open the content instead of Excel. So there is no good workaround for this case because of the lack of a special MIME type for HTML/MHTML that is Excel specific. You can add your own MIME type if you control both the web server and the client desktops that need access to it but, otherwise the best option is to use a different file format or alert your users of the warning and tell them to select Yes to the dialog.
Another Workaround :
Client Configuration of Extension Hardening.
Client desktops can change the default behavior of the security check using a registry key. The following registry key can be set to alter the behavior ofthe Excel 2007 client in the following ways:
Key: HKEY_CURRENT_USER\Software\Microsoft\Office\12.0\Excel\SecurityValue: (DWORD)"ExtensionHardening" = [0 = Disable check; 1 = Enable check and prompt; 2 = Enable check, no prompt deny open]Default setting if value not present is 1 (enable and prompt).
The client can configure the site to disable the check by setting ExtensionHardening = 0. This will stop the prompt from appearing, but it will also disable any future checks for the file content validity. This does not (in itself) lower your security. The check is part of a defense-in-depth strategy to prevent malicious users from crafting files of a different type than its extension (or MIME type), which could then be used to trick users into opening a file of a type they did not expect. This warning is designed to let the user know of this potential threat and given them an option of canceling the open if they wish.