I see. But then, if form1 uploads the file it is already on the server.
What would be the preferred method of passing the file's name to form2?

Again, this is because you cannot set a default value (by preserving) 
of a file field for security reasons.

I whish to process a series of forms in a minor variation to 
https://wiki.horde.org/Doc/Dev/FormPackage. Form variables in a step 
are saved in the $vars object to process the successive step. All 
works well unless the variable is of type file.

I included a demo script that illustrates the point. If FormStage1 
uses a variable of type integer everything works. However, if the type 
is file the gathered information is an array and preserve() fails:

dumping vars object:
Array
(
     [f_file] => Array
         (
             [name] => parsedFile-5.docx
             [type] => 
application/vnd.openxmlformats-officedocument.wordprocessingml.document
             [tmp_name] => /tmp/phppIjQKh
             [file] => /tmp/phppIjQKh
             [error] => 0
             [size] => 35069
         )

     [stage] => 2
     [f_bool] =>
)