Flat File Importer should allow alternate file extensions

In the Flat File Importer (on the TeiidMetadataImportSourcePage) the user selects a folder, then the 'available' files in the folder are shown for selection.

Currently, files of form .txt, .csv and (no ext) are shown - and this is hard-coded. I would recommend adding a filter drop down (or ?) to show the assumed defaults, but then allow the user to change the filter to "." so that all files are shown.

We should not force the user to name their data files a certain way. For example if they have .dat files, there is no way to see them currently.