Fix file downloader algo duplicating file's extension

[nicogodet]

Fixes #60528

User can provide a properly named temporary filepath (filename + extension) and the logic applied in current alg is not necessary and can create issues if this alg is called in a larger process: the output specified miss match the filename created on disk

[github-actions]

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[alexbruy]

Could you please add test?

[nicogodet]

@alexbruy I can. Is it OK to use QgsProcessingUtils::generateTempFilename() in tests ?
I see other test use QDir::tempPath() + "/filename.ext"
To properly test this change, it would be better to use QgsProcessingUtils::generateTempFilename()

Test will be added to existing one:

QGIS/tests/src/analysis/testqgsprocessingalgspt2.cpp

Line 1129 in 0b9bc85

void TestQgsProcessingAlgsPt2::fileDownloader()

  const QString outputFileName = QgsProcessingUtils::generateTempFilename( QStringLiteral( "qgis_version.txt" ), *context );
  parameters.insert( QStringLiteral( "OUTPUT" ), outputFileName );

  results = alg->run( parameters, *context, &feedback, &ok );
  QVERIFY( ok );
  // compare result filename is the same as provided
  QCOMPARE( outputFileName, results.value( QStringLiteral( "OUTPUT" ) ).toString() );

I'm not a c++ dev...