The default "Password Protected Files" attachment list does not contain 7z format.
Steps in order to obtain custom list with 7z format included are the following:
a) Go to Content/Resources/Attachment lists
b) Mark default password protected files and then click "Clone"
c) Put the name that best reflects your new attachment list, and then click "Add Attachment Types", mark " Password Protected" then search for "Ecapsulation Format" and select "7 zip format (7z)" from "File Types" - mark and click "Add".
d) Check "Attachment Types" section and confirm that 7z has been added. Once done please save.
Following the same instruction, other file types can be added.