File Upload settings
This setting controls the behaviour of file upload field in the form. You can access it from Global Settings >> General Settings. It comprises of following two sub-settings:
Allowed File Types
It is a text area that allows you to define file extensions, permitted to be uploaded by the user with form submission, if you have a “File” field in your form. Comes handy if you want to accept only certain type of files, for example images or documents. Each extension has to be separated by a pipe (“|”) symbol. Following is a value for allowing common image types:
This setting can be overridden on per-field basis. When you create a “File” field, you can specify the allowed types in the field settings as well as those that will override global settings.
Note that: There are only certain file types allowed by WordPress itself. Link : https://codex.wordpress.org/Uploading_Files#About_Uploading_Files_on_Dashboard
Allow Multiple Attachments
A toggle switch that turns on the feature to allow user submit more than a single file with a single file field in the form.