Hi Subh,
I have an idea as follows,
You are using non persistable entities before committing that a good thought, use one more attribute in the page context entity as Error message.
Use this under the File dropper widget as text box, fill the same attribute in the validating microflow to write as "File format not allowed"+$File Name to specify the each file.
If all validations passed that error message would be empty, use the visibility as $ErrorMesssage != empty.
Hope this helps!
Regards,
Sathya Reddy