how we can fetch the document extension name and document type in when we upload something on UI
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Unfortunately not possible - see full documentation here: https://docs.appian.com/suite/help/23.3/File_Upload_Component.html#uploaded-files-are-stored-as-temporary-documents-until-submission
"When a file is uploaded, the component updates with a temporary document representing the uploaded file. Between the upload and the submission, the file on the server is an inactive temporary file and not accessible except through fv! variables described in the File Name, File Description, and Validations parameters. This is the only way to access file metadata before the file upload is submitted."
fv!