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Your LimeSurvey version: 6.5.9
Own server or LimeSurvey hosting: Own server
Survey theme/template: Fruity
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I want to link to the file that was uploaded by the user in the previous question.
I inserted the following code in a text question (Type X).
<a href="{G01Q02.file}">Click here to download</a>
But when I run the survey I see the message "Unknown variable"
 

Hi, you have to consider several things.
1. {G01Q02.file}
".file" is not a LimeSurvey property.
See here
[url] manual.limesurvey.org/ExpressionScript_-...#Access_to_variables [/url]

2. When you only display the content of {G01Q02} you see something like this
 
Among others there is a "name"; this is the original name of the uploaded file. And there is a "file-name" ("fu_" and some random characters)
And if you have a look at the "files" folder of the survey you will see that the files are stored with this "file-name" without extension.
 

Meaning you can't open this file out of the box.

A way I described in my "Tutorial 3: Gimmicks", Chapter 16.2. f. is to rename this "fu-..." file to a usable name 
You find it in the German part, page 13.
BTW: Now I think it to be better to rename to the original name and store in a different folder.

I used an ajax call like this
And in a small php script "renameFile.php" I renamed the file and returned the new name.
Now you can use it to show the file.

Upload
 

And a later display.
 

Joffm
 

Thank you!
Best regards,
Marc.