File upload form field is not working

when you add the file field to the comments form there will be a javascript error indicating that the dropzone previewTemplate is not set. If you fix this then more js errors occur.
But the file field inside a contact form is working very well.

Start by reproducing the file field in the comments form and compare its JavaScript setup with the working file field in a contact form. Trace the reported Dropzone previewTemplate error and subsequent JavaScript errors; the work is done when file uploads in the comments form work without those errors.

Written by the indexing model from the issue text.