This is not a problem - I found a solution. I thought I should share it with the community.
Goal: provide a webform with a file upload field, and then convert the submitted entry to a Post. In the post, show the uploaded file in a custom download box with a download icon, or if there was no file upload submitted, do not show the download box.
Solution: Setup 5 fields in the form:
(1) a Yes/No radio button field for "Upload a File?" set to "No" default;
(2) a Custom field set to Upload File, with a condition to display if #1 is Yes;
(3) a Custom Field set to Admin view only with a default value set to "upload" (without the quot marks and no period before it - this is a custom CSS I created in the master style sheet that forms the Download box and icon). This field is set to a condition to display if #1 is "Yes", but since it is an Admin Only field, it doesn't actually display in the form to the user;
(4) Another Custom Field, same as #3, except with a default value of "hide" (a CSS class set to display:none) and a condition to display with #1 as a "No".
(5) a Post Body field, set to Admin Only, with Merge tags from other fields, including the following
<div class="{Admin Only - Hide upload if none selected:26}{Admin Only - Display if file uploaded:27}"><a href="{File upload:20}"><img src="http://mysite.com/wp-content/uploads/textdoc_tn.png" /><span>Download Supporting Documentation</span></a></div><br /><p>[embed]{Post your YouTube link here:23}[/embed]</p>
The trick: The merge tags above for Custom Fields #3 and #4 are set to insert either the "upload" or "hide" class into the display div class depending on which Yes/No selection is made. Note that the default value in #3 and #4 is just the class name WITHOUT the period in front of it.
BTW, the [embed] section is a single line text field for users to copy/paste a YouTube URL into it, which publishes it in the post. Even though WP has the embed parsing capabilities built-in, it doesn't work without these shortcode tags added around the merge tag.
If there is a more poetic way of doing this, please add a comment. I tried a LOT of other solution before this worked!
