With the File Input, you can either put a url in the text box to fetch the file from or click the upload file button to use a file from your device. If you upload a file, the name is put in the text box; if the text box contents are changed or a new file is uploaded, the old file is deleted.
Be mindful while using the web client that Sourcetab can only store up to 5MB of data. Either upload small files or only use hosted files from the web through the url text box.