You can't; this is due to security/privacy concerns.
What you want to do is upload the file and save it to a "temp" location.
Then you do your validation routine, if it passes you then send it to the final location.
There's a pretty good answer on SO for this; so I am not going to reinvent the wheel.
c# - Validate Excel(Spreadsheet) File Using EPPlus - Stack Overflow[
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