diff --git a/build/lib/extensions.js b/build/lib/extensions.js
index c6d56319c30..4e704bdab6f 100644
--- a/build/lib/extensions.js
+++ b/build/lib/extensions.js
@@ -158,10 +158,12 @@ function fromLocalWebpack(extensionPath, webpackConfigFileName, disableMangle) {
                     // source map handling:
                     // * rewrite sourceMappingURL
                     // * save to disk so that upload-task picks this up
-                    const contents = data.contents.toString('utf8');
-                    data.contents = Buffer.from(contents.replace(/\n\/\/# sourceMappingURL=(.*)$/gm, function (_m, g1) {
-                        return `\n//# sourceMappingURL=${sourceMappingURLBase}/extensions/${path.basename(extensionPath)}/${relativeOutputPath}/${g1}`;
-                    }), 'utf8');
+                    if (path.extname(data.basename) === '.js') {
+                        const contents = data.contents.toString('utf8');
+                        data.contents = Buffer.from(contents.replace(/\n\/\/# sourceMappingURL=(.*)$/gm, function (_m, g1) {
+                            return `\n//# sourceMappingURL=${sourceMappingURLBase}/extensions/${path.basename(extensionPath)}/${relativeOutputPath}/${g1}`;
+                        }), 'utf8');
+                    }
                     this.emit('data', data);
                 }));
             });
diff --git a/build/lib/extensions.ts b/build/lib/extensions.ts
index 2a9a215c5e4..14f2de9fef5 100644
--- a/build/lib/extensions.ts
+++ b/build/lib/extensions.ts
@@ -170,10 +170,12 @@ function fromLocalWebpack(extensionPath: string, webpackConfigFileName: string,
 						// source map handling:
 						// * rewrite sourceMappingURL
 						// * save to disk so that upload-task picks this up
-						const contents = (<Buffer>data.contents).toString('utf8');
-						data.contents = Buffer.from(contents.replace(/\n\/\/# sourceMappingURL=(.*)$/gm, function (_m, g1) {
-							return `\n//# sourceMappingURL=${sourceMappingURLBase}/extensions/${path.basename(extensionPath)}/${relativeOutputPath}/${g1}`;
-						}), 'utf8');
+						if (path.extname(data.basename) === '.js') {
+							const contents = (<Buffer>data.contents).toString('utf8');
+							data.contents = Buffer.from(contents.replace(/\n\/\/# sourceMappingURL=(.*)$/gm, function (_m, g1) {
+								return `\n//# sourceMappingURL=${sourceMappingURLBase}/extensions/${path.basename(extensionPath)}/${relativeOutputPath}/${g1}`;
+							}), 'utf8');
+						}
 
 						this.emit('data', data);
 					}));