Hello,
im having an issue with making my Calc locked to one certain extension (in this case .xls). I don’t know is it even possible bcs I searched lot of web for the answers and haven’t found one has similar question/issue.
Idea is that I send my .xls document to other, they fill the needed blank cells and return me the document in the same format. I put the default saving extension in options to .xls but there is always someone who change the extension to .xlsx, .ods., etc. by themselves. Users are using different software and there is lot of them. So, if anyone has an answer to my prayers or workaround I would be really thankful because deadlines are approaching. 
BR,
G