In our extranet environment when a user tries to open an office file a message pops up:"The file you are trying to open, 'filename.aspx' 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 wan to open the file now?" If they click yes an .aspx file opens in the program displaying the login screen.Any ideas?

I can't say I know for sure, but if this is an office file it should be .doc, .docx, .xsl or .xslx and so on. Sounds to me that when someone uploaded the document they changed the extension to .aspx which is a web page, not an Office file. This is happening for all files?