diff --git a/app/components/image/upload/image-uploader.ts b/app/components/image/upload/image-uploader.ts
index 15ae6862eb..17e8c14c51 100644
--- a/app/components/image/upload/image-uploader.ts
+++ b/app/components/image/upload/image-uploader.ts
@@ -61,7 +61,8 @@ export class ImageUploader {
         }
 
         const files = ImageUploader.getFiles(event);
-        uploadResult.messages = uploadResult.messages.concat(this.checkForUnsupportedFileTypes(files));
+        const message: string[]|undefined = this.checkForUnsupportedFileTypes(files);
+        if (message) uploadResult.messages.push(message);
 
         const imageFiles = files.filter(file =>
             ImageUploader.supportedImageFileTypes.includes(ExtensionUtil.getExtension(file)));
@@ -86,7 +87,7 @@ export class ImageUploader {
     }
 
 
-    private checkForUnsupportedFileTypes(files: Array<File>): string[] {
+    private checkForUnsupportedFileTypes(files: Array<File>): string[]|undefined {
 
         const supportedFileTypes = ImageUploader.supportedImageFileTypes.concat(ImageUploader.supportedWorldFileTypes);
         const result = ExtensionUtil.reportUnsupportedFileTypes(files, supportedFileTypes);
@@ -97,7 +98,7 @@ export class ImageUploader {
                 supportedFileTypes.map(extension => '.' + extension).join(', ')
             ];
         }
-        return [];
+        return undefined;
     }
 
 
diff --git a/app/core/util/extension-util.ts b/app/core/util/extension-util.ts
index c2a8a38592..fdcb8df886 100644
--- a/app/core/util/extension-util.ts
+++ b/app/core/util/extension-util.ts
@@ -24,7 +24,7 @@ export class ExtensionUtil {
         let result: Array<any>
             = [(files.length - ExtensionUtil.getUnsupportedExts(files, supportedFileTypes).length)];
         if (uniqueUnsupportedExts.length > 0) {
-            result.push(uniqueUnsupportedExts.join(','));
+            result.push(uniqueUnsupportedExts.join(', '));
         }
         return result;
     }