Merging variable with string in config file
This works:
set $foo = 'foo';
set $foobar "${foo}bar";
Nginx does not have a concatenation character, rather it's based on valid and invalid characters, for instance in the directive:
try_files $uri $uri/ @fallback;
$uri is the variable and / is a string to append since / cannot be in a variable name.
Similarly you should try
$upload_field_name[name] "$upload_file_name";
If this doesn't work then try.
set $foo [name];
$upload_field_name$foo "$upload_file_name";
I cannot say if the upload module will even allow this, though. Minor syntax errors might also be present.