To save a file in relation to a contact, you must select "Required Login".   Choose the object you wish to associate the file with.  In this scenario I want the uploaded files to be related to the contact record, so we'll choose contact.   (You could however use ARS and associate a file with a connection, or with any other object)

Choose populate fields from Salesforce is you want the user to be able to access the files that they've uploaded.  

Drag the File Upload field onto the form layout and configure it:

1) Determines is uploading the file is required.

2) Private: Don't set to private unless you want only user and CMS Admin to be able to access the file. It will not be visible in Salesforce if synched.    This cannot be reversed if files were previously uploaded as private, so not set to Private without fully considering your need to access these files.

3) Is File an Image?  This is optional, but allows you specify a recommended width and recommended height for the image.  This value is not enforced however.  

4) Save to Salesforce:  Select this if you wish the file saved to Salesforce

5) Salesforce Library - Salesforce can optionally save files in a library.  You do not have to choose a library or create one, in which case it will be put in the default Salesforce Library. If you wish to create specific libraries for certain types of file uploads you can do so.  See this article from Salesforce Help on how to create and manage libraries.

6) Store as Related Files:  Setting this to 'yes' will associate the file with the record you are creating or updating in Salesforce.  In our example here, we are synching to the contact record (of the logged in user), so the file will be available as a related file to the logged in contact.

7) Extensions Allowed:  specify all the file types you are allowing to be uploaded.  You must select at least one to save the field and the form.  (Otherwise they can't upload anything!).  This allows you to limit uploads to documents, images, or whatever is appropriate.  (if Is file an Image is selected, you can only choose extensions for images).

8) Don't populate from Salesforce:  When checked the files associated with this contact will not be visible for the user to delete or edit.

Create a page for your form, and put in any helpful instructions.  Users can drag & drop or browse files to upload, and can upload multiple files at once.   After all files are uploaded, they would click Submit to finish the form and the files will be submitted.

Subsequently - a logged in user will have access to the files they've uploaded (if you've selected "Populate Fields from Salesforce".