The easies way to do this using SQL Server 2017 and up;
DECLARE @FileName NVARCHAR(255) = N'Need to get extension of this file.xlsx';
--PRINT @FileName
SELECT
@FileName ExcelFileName,
--check there is a '.' in ExcelFileName
CASE WHEN @FileName LIKE '%.%' THEN
REVERSE(left(REVERSE(@FileName), CHARINDEX('.', REVERSE(@FileName)) - 1))
ELSE ''
END ExcelFileExtension
WHERE 1=1
Earlier versions can use this approach;
DECLARE @FileName NVARCHAR(255) = N'Need to get extension of this file.xlsx';
--PRINT @FileName
SELECT
@FileName ExcelFileName,
CASE
WHEN @FileName LIKE '%.%' THEN
RIGHT(@FileName, LEN(@FileName) - CHARINDEX('.', @FileName))
ELSE ''
END ExcelFileExtension
WHERE 1=1
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