Fixes U4-483 - Replace illegal characters in media with a dash, not underscore

Also removing multiple repeating dashes, leaving only one in place.

src/Umbraco.Core/IO/IOHelper.cs

@@ -222,7 +222,7 @@ namespace Umbraco.Core.IO
             {
                 foreach (var character in Path.GetInvalidFileNameChars())
                 {
-                    filePath = filePath.Replace(character, '_');
+                    filePath = filePath.Replace(character, '-');
                 }
             }
             else
@@ -242,10 +242,14 @@ namespace Umbraco.Core.IO
                 if (reservedCharacters.IndexOf(character) == -1)
                     stringBuilder.Append(character);
                 else
-                    stringBuilder.Append("_");
+                    stringBuilder.Append("-");
             }
 
-            return stringBuilder.ToString();
+            // Remove repeating dashes
+            // From: http://stackoverflow.com/questions/5111967/regex-to-remove-a-specific-repeated-character
+            var reducedString = Regex.Replace(stringBuilder.ToString(), "-+", "-");
+
+            return reducedString;
         }
     }
 }