Fix run files widget to use set property name. Refs #5700

It had been accidentally hard coded to use Filename as a property name
when checking if a file is valid for an algorithm.

Code/Mantid/MantidQt/MantidWidgets/src/MWRunFiles.cpp

@@ -143,9 +143,10 @@ void FindFilesThread::getFilesFromAlgorithm()
   if(!algorithm) throw std::invalid_argument("Cannot create algorithm " + m_algorithm.toStdString() + ".");
 
   algorithm->initialize();
-  algorithm->setProperty("Filename", m_text);
+  const std::string propName = m_property.toStdString();
+  algorithm->setProperty(propName, m_text);
 
-  Property *prop = algorithm->getProperty(m_property.toStdString());
+  Property *prop = algorithm->getProperty(propName);
 
   FileProperty *fileProp = dynamic_cast<FileProperty*>(prop);
   MultipleFileProperty *multiFileProp = dynamic_cast<MultipleFileProperty*>(prop);
@@ -156,7 +157,7 @@ void FindFilesThread::getFilesFromAlgorithm()
   }
   else if( multiFileProp )
   {
-    std::vector<std::vector<std::string> > filenames = algorithm->getProperty("Filename");
+    std::vector<std::vector<std::string> > filenames = algorithm->getProperty(propName);
     std::vector<std::string> flattenedNames = MultipleFileProperty::flattenFileNames(filenames);
 
     for( auto filename = flattenedNames.begin(); filename != flattenedNames.end(); ++filename )