Update xls_available key to accommodate different file extensions

[faith-mutua]

Environmental Information

• Onadata version:

Problem description

The xls_available key in the XForm instance currently supports two file formats i.e. .xlsx & .xml which results in the UI defaulting to .xlsx for all forms even those with the .xls extension. We need this key updated to create provision for the three XLSForm (.xls, .xlsx, .xml) formats to be available. The updated key will make it possible to use the extension of the actual file instead of assuming that all XLSForms are of the form .xlsx.

Steps to reproduce the behavior

1. Upload a form with a .xls extension
2. Navigate to the form settings tab
3. Observe that the form Source component has the .xlsx extension added to the the form id string

Additional Information

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