Repro: 1. Save a JPEG on your desktop as foo.Jpg 2. Drag drop file to Fiddler Observe: - File does not have correct Content-Type response header. File extensions should be treated as case-insensitive when mapping to MIME types (Utilities::ContentTypeForFileExtension). 3. Rename a text file to foo.map 4. Drag drop file to Fiddler Observe: - File does not have correct Content-Type response header from the registry. It looks like the registry may not be getting consulted properly; perhaps this was done with the introduction of CustomMimeMappings.xml which was presumably added to better support running on Mono/Linux, but Fiddler should still fall back to the Windows registry on Windows.