Nestjs throw error and http status 400 when fileFilter in fileinterceptor finds an unsupported file extension

I am having difficulty handling an error in my application.

I want to create a FileInterceptor (multer) which checks if the file has an allowed file extension. The documentation shows this:

const allowedFileExtensions = ['.jpg', '.png'];

FileInterceptor(
  'image',
  {
    dest: './uploads',
    fileFilter: (req, file, callback) => {
        const extension = path.extname(file.originalname);
        if (allowedFileExtensions.includes(extension)) {
           callback(null, true);
        } else {
           // gives the 500 error
           callback(new Error('Only images are allowed'), false);
        }
    }
}

This kinda works. But it has two flaws in my opinion. First, it returns a 500 error:

{
    "statusCode": 500,
    "message": "Internal server error"
}

It would be better to return a 400 error with a error message which explains why it failed.

Second, in the console of the Nest application it shows the stacktrace. I'd rather use the logging for this instead. (The application has middleware in place to pick up NestJS errors and log them automagically.)

So what I am trying is the following:

const allowedFileExtensions = ['.jpg', '.png'];

FileInterceptor(
  'image',
  {
    dest: './uploads',
    fileFilter: (req, file, callback) => {
        const extension = path.extname(file.originalname);
        if (!allowedFileExtensions.includes(extension)) {
            // crashes the application
            throw new BadRequestException('Only images are allowed', `Bad request. Accepted file extensions are: ${allowedFileExtensions.toString()}`);
        }
        callback(null, true)
    }
}

But this crashes the application. I am not able to upload a different image anymore after this.

Any idea?

Ah found it. The BadRequestException cannot be thrown outside the body of the function.

const allowedFileExtensions = ['.jpg', '.png'];
enum FileValidationErrors {
    UNSUPPORTED_FILE_TYPE
}

@Post('/:id/upload-photo')
    @UseInterceptors(
        FileInterceptor(
            'image',
            {
                dest: './uploads',
               fileFilter: (req, file, callback) => {
                 const extension = path.extname(file.originalname);
                 if (allowedFileExtensions.includes(extension)) {
                    callback(null, true);
                } else {
                    // provide the validation error in the request
                    req.fileValidationError = FileValidationErrors.UNSUPPORTED_FILE_TYPE
                    callback(null, false);
                }
              }
            }
        )
    )
    uploadSinglePhoto(
        @Param('id', ParseIntPipe) id: number,
        @UploadedFile() image,
        @Req() req // add the request property
    ): Promise<UserProfileEntity> {
        // check here for a potential error
        if (req?.fileValidationError === FileValidationErrors.UNSUPPORTED_FILE_TYPE) {
            // if so, throw the BadRequestException
            throw new BadRequestException('Only images are allowed', `Bad request. Accepted file extensions are: ${allowedFileExtensions.toString()}`);
        }
        this.logger.verbose(`Adding image ${image}`);
        const imageUrlLocation = `${image.destination.substring(1)}/${image.filename}`;
        return this.userProfileService.saveUserProfilePhotoLocation(id, imageUrlLocation);
    }