Validating on upload is sensible and should be done anyway.
However, left-padding is not always going to work, it depends on the data.
Common problems I see are with phone numbers. UK mobiles begin 07nnn. Excel kindly drops the leading zero and then really helpfully treats the long number as being an exponential-format floating point number, which cannot be turned back to the original number.
In our systems, we require country codes in front of the numbers, so UK mobiles should being 447nnn. That doesn't lose the leading zero but still gets the E-format mess when opened by Excel. Customers often miss out the 44 and use the national 07 format. If that leading zero is missing, it can be quite difficult for a system to reliable determine whether that number, beginning with a 7, is an international number for a destination in Russia, or a user error and should've been 447, from an 07 data value.
Because Excel is a spreadsheet, for calculating with numbers.
Then there's additional code for dealing with all the inconvenient ways people format things, or want to add text labels, or do things closer to numerical/financial analysis, or all the other extras wrapped around the core "put numbers in boxes and do math".
That misunderstanding is at the core of Excel misuse.
