Use correct file extensions with inserted files (#1385)

* Fix boolean logic for finding file extensions
* add test for correct file extension when adding ImagePart. Fixes #1305

Author: mikeebowen

src/DocumentFormat.OpenXml/Packaging/OpenXmlPart.cs

@@ -134,7 +134,7 @@ private ITargetFeature GetAndVerifyTargetFeature(OpenXmlPackage openXmlPackage,
 
             if (!IsContentTypeFixed)
             {
-                if (!openXmlPackage.PartExtensionProvider.TryGetValue(contentType, out var found))
+                if (openXmlPackage.PartExtensionProvider.TryGetValue(contentType, out var found))
                 {
                     targetExt = found;
                 }

test/DocumentFormat.OpenXml.Tests/ofapiTest/OpenXmlPartTest.cs

@@ -258,7 +258,11 @@ public void AddPartTest()
                     doc.AddMainDocumentPart();
                     var document = doc.MainDocumentPart.Document = new Document();
                     document.Save();
+
+                    // ImagePart has correct file extension
                     var imagePart = doc.MainDocumentPart.AddImagePart(ImagePartType.Jpeg);
+                    Assert.True(imagePart is not null && imagePart.Uri.OriginalString.EndsWith(".jpg"));
+
                     var extendedSubPart = doc.MainDocumentPart.AddExtendedPart("http://temp", "text/xml", ".xml", "tempId");
 
                     var extendedPart = doc.AddExtendedPart("http://temp", "text/xml", ".xml", "tempId");