referencing dynamic field names

I'm having an issue testing a form field for a null condition.  The field names on the form are created dynamically as the form is created based on a query that returns a list of documents uploaded by a user. Each row will contain the document ID, document title and file name. As I loop through the query I create the form field name for each row by attaching the document ID to a string. Here is the code that creates the form:

<cfform action="ec_process_attachment_upld.cfm"  method="post" enctype="multipart/form-data">  

<cfoutput>

<cfif #get_attachment_list.RecordCount# GT 0>

       <table border="1">

       <cfloop query="get_attachment_list">

             <cfset f_name = "upld_attachment_file" & #attachment_id#>

             <tr>

                    <td>#attachment_name#</td>

           <td><input type="file" name="#f_name#" value=""></td>

           <td>Delete</td>

       </tr>

       </cfloop>

       </table>

</cfif>

</cfoutput>

So, the field names generated will be upld_attachment_file1, upld_attachment_file2, etc.

Here is the code that does the processing of the form:

<cfloop query="get_attachment_info" >         

       <cfset f_name = "upld_attachment_file" & #attachment_id#>

       <cfset t_var = "form." & "#f_name#">

       <cfif len(#t_var#) GT 0>

             Do stuff

       </cfif>

  </cfloop>

The query here is the same and the one that created form. As I loop through it, I'm again creating the field name on the fly. The problem comes in the CFIF statement when I try to test to see if the field is null or not. The test ends up testing the variable that hold the form field name rather than the form field itself. How can I test the form field?

Thanks

DW