CURRENT SITUATION
Currently, when opening an Excel file from Proton Drive in Proton Sheets, a conversion takes place that makes a copy of the file with filetype 'application/vnd.proton.sheet'.
That means you now have two different versions of the same file, one of which can only be edited in Proton Sheets and one of which can be used in any other spreadsheet application.

SUGGESTION
I am aware that Proton Sheets can be exported back to the XLSX format, but I want to request first-class support for XLSX files without conversion. Reasons include:
- Being able to edit my XLSX files locally using the software of my choice (especially relevant when using Proton Drive sync).
- Also being able to edit my XLSX files from Proton Drive when pulling them local is inconvenient (e.g. on a shared computer) without generating semi-duplicate files.
- Reducing the chance of malformatted XLSX files. In theory, conversion shouldn't break formulas, formatting, etc. In practice, this is never without flaws (in my experience).
- Preventing vendor lock-in. Google Sheets has their own proprietary fileformat, but they also support editing XLSX files without conversion, which ensures that (if you use that) you can copy-paste the contents of your drive to anywhere else, and not lose anything.
- Promoting & keeping in line with industry standards. XLSX is the de-facto standard for spreadsheets. ODS exists, but is less commonly used (though first-class support for that would also be great).