JotForm is a free online form builder which helps you create online forms without writing a single line of code. No sign-up required.
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Unfortunately, we do not have a feature or widget that allows you to directly feed a drop down field on JotForm from a text file as a source. You'll need to manually add the list of items in your form.
We do have some drop down widgets that you can check. You can search them on this page: http://widgets.jotform.com/. We have the "User Contributed Drop down" widget that allows your users to add an option in the drop down field.
Another possible way is to use the form's full source code, create a custom script that will prepopulate your options list from a text file source. However, you'll need to embed the form's source code on a web page/server and you'll need to develop or hire a developer that will create the custom script.
Here are some discussions about it: http://stackoverflow.com/questions/14857054/populating-drop-down-menu-from-comma-delimited-text-file?answertab=active#tab-top.
You can try loading the text file using JSON.
I hope that helps.