Hi
One option would be as part of your page to call a power automate flow, pass it the details of the file, (if you need to temporarily save it in SharePoint or other secure location while doing your validations, then delete it).
While you could also save it to a working table, which would trigger a power automate flow and do validations, it would not come back to your page, it would have to tell them later.
So you would let them know you are doing verifications and they will hear back shortly
Save it into working table
Flow triggers
It does the validations etc
if all good it saves it in the table and updates some tracking data in your working table, or deletes it
If it doesn't work, it would email you and them, letting them fix and retry later.
Technically you could trigger a flow in your page, but i'd have to see the page and how you are creating it to say exactly how and you want to make sure its can take the file data, save it temporarily (as above suggestion), do the work in real-time, then let you know if it worked or not and if not, give you back the list of issues, which you give back to the user and wait until they retry.