add uploaded_file_name function

[djyotta]

Closes #333

Tested with

SET fname = sqlpage.uploaded_file_name('f');

SELECT 'debug' AS component;
SELECT $fname AS fname;

SELECT 'form' AS component, 'post' AS method;
SELECT 'file' AS type, 'f' AS name, '' AS label;

Documentation tbd

[djyotta]

Test and documentation added

[lovasoa]

I don't think we can expect the SQLPage documentation readers to be familiar with python

[djyotta]

I don't think we can expect the SQLPage documentation readers to be familiar with python

I've modified the docs to demonstrate download via data uri instead of the python example.

[lovasoa]

Merged! Thank you again, @djyotta !