If you prefer to still have multiple upload field buttons, you can add one using HTML.
Tip: Add all the form fields you need first, once you go into custom mode to add the additional fields, you'll save it while in custom mode.
Go to your customize section of your offer. Click anywhere in the general area of your form, that will populate a menu on the left side of your page. Add all the form fields you need in general.
When you're satisfied with your form and are ready to add your additional upload field, you'll click "code" on the furthest left sidebar. Next, select "Edit HTML". You'll see the HTML code appear to your right. Find the div class code for "fileupload". You're going to want to copy (ctrl+c) from the beginning div class to the last closing /div before the next div class starts. Look at the example below for reference.
The next step is to paste (ctrl+v) it where you want it in the form. If you want it below the first upload button, then just add it after the last /div closing, like the screenshot below.
Note: You must change the "Name" and "ID" of the second file upload. You can just change the custom_number to any number that is not already being used, or add a letter to differentiate the fields for your export. Do not use spaces.
You can also change the "label" of your additional fields by changing the black text that says "Upload a Photo". You can change it to "Upload a second photo" or anything specific you need... though it is not necessary, they can be labeled the same if you wish. After you've set it up, just click "close editor" and then "save changes" both populate on the top right corner.
That's all you need to do. You can paste the code for each additional upload field you want to add.