How do I add a Custom Field in the forms eg., a textbox, or a field to let attendees upload files?

Go to admin backend -> Login ->
Click on Custom Field button located in second menu -> click on Add
New Field button -> Select Field Type , Fill Name and Description,
select Include in (Where you want to include this field) , choose if field
will be mandatory or not -> click on save button.

Adding profile photo of contacts

 To add a profile picture of a contact, go to Contacts. Click on the name of the contact (search for the contact by name or ID if you do not see it in the current list) whose photo you want to upload/replace. On the left side, you shall see a default picture with a browse button. Click on browse button, choose photo from your computer, and click on Save on the top right. That’s it.