Excel version 5 is hardly less ancient ;)

Pretty much all Office files in formats prior to 2007's .xlsx, .docx etc. that one may encounter in the wild are malware vectors. I can't remember one that was NOT malware. They are blocked by all security-minded mail admins and are increasingly blocked by major service providers and platforms. It doesn't surprise me in the slightest that the likes of Apple would refuse to open them.

A more resilient solution is to output to .xlsx. I seem to recall FoxyPreviewer can do that, though I've never used it. I have used Greg Green's class: https://github.com/ggreen86/XLSX-Workbook-Class . It has a bit of a learning curve and is not trivial to use but it gives complete control. For one task I used it to create and populate multi-worksheet workbooks from scratch.

>Don't use XLS, it's a very old file format. Try XL5 instead.
>
>>Hello all,
>>
>>I wish you all to have a healthy and happy new year 2023!
>>
>>In my application I export some data to an excel file. The problem is that when I look at that created Excel file within the file explorer on my conputer everything is fine. The thing is that I open the file on my iPhone I receive an error message telling me that it can't be previewed.
>>
>>I export this way
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>>EXPORT TO FileExported TYPE XLS
>>
>>Is it only a problem of the extension XLS?
>>
>>I noticed that there is a class available to manage excel files. Would using it fix my problem?