I'm new to creating extensions for Dreamweaver. I found a PDF document entitled "Packaging Extensions with
ADOBE® EXTENSION MANAGER CS5" and it seems to indicate that you need to create a ZXP file when creating an extention for CS5 and above. Does this apply to Dreamweaver extensions? Are all of the extension gurus out there creating ZXP files or are they creating MXP files for Dreamweaver CS5 and up extensions? Here is a link to the PDF document: http://help.adobe.com/en_US/extensionmanager/cs/using/packaging_extens ion.pdf
If your extension is only for DW5 or above then create a ZXP file as this will prevent anyone with an older versions of DW installing the extension. But if it will work in previous version then MXP is the file to create, as this can be installed in all versions of DW including DW5 or above.
When submitting your extension to the Adobe Exchange or even if you only offer it as a download from your own site, always indicate the version you are targeting and/or if it will work/has-been-tested in a previous version.