FileBarrier.Core is a simple little library (for .NET Core >= 2.1) built to help you to check if the uploaded file is meeting the validation requirements you have by providing whitelist to it. It is an extension method on IFormFile.

Download the package form NuGet FileBarrier.Core. Current release is 1.0.5.

This library gives you different ways to check if the file is valid or not. You can check for:

• CheckExtensions: When you want to check for the file extension against a whitelist of the allowed file extensions (provided by you the developer).
• CheckFileContentTypeToFileExtension: This will check if the MIME-type of the provided file is correct for the file extension, then it will check against a whitelist of the allowed MIME-types (provided by you the developer).
• CheckContentType: will check if the MIME-type of the provided file is in the a whitelist of the allowed MIME-types (provided by you the developer).
• CheckFileSize: will check if the file size is <= a specific size (provided by you the developer).
• AllLayers: will check for all above layers.

Things you need to provide depends on the layers you want:

• If you want to use the CheckExtensions layer: then you need to provide allowedExtensions a string contains the allowed extensions comma-separated, e.g: "txt,pdf,doc,docx", otherwise an ArgumentNullException will be thrown.
• If you want to use the CheckFileContentTypeToFileExtension layer: then you need to provide allowedContentTypes a string contains the allowed extensions comma-separated, e.g: "text/plain,application/pdf,application/msword".
• If you want to use the CheckContentType layer: then you need to provide allowedContentTypes a string contains the allowed extensions comma-separated, e.g: "text/plain,application/pdf,application/msword", otherwise an ArgumentNullException will be thrown.
• If you want to use the CheckFileSize layer: then you need to provide maxSingleFileSize the maximun file sieze, otherwise an ArgumentNullException will be thrown.
• If you want to check for AllLayers: then you need to provide allowedExtensions, allowedContentTypes, and maxSingleFileSize.

After providing the required information, then you can check if the file is allowed or not by calling either:

• IsFileAllowed: returns boolean to indicates if the file is allowed or not against the selected layers. E.g.:

...
private const string ALLOWED_MIME_TYPES = "image/png,image/jpeg";
private const string ALLOWED_EXTENSIONS = "png,jpeg";
public IFormFile File { get; set; }
...

public void OnlyPngOrJpeg(){
    var isAllowed = File.IsFileAllowed(ALLOWED_EXTENSIONS, ALLOWED_MIME_TYPES, null, FileCheckLayers.CheckExtensions, FileCheckLayers.CheckExtensions, FileCheckLayers.CheckFileContentTypeToFileExtension, FileCheckLayers.CheckContentType);
    if(!isAllowed) {
      //do whaterver you need to do.
    }
}

• IsAllowed: returns BarrierResponse to give you more information about on which layer the checking has occurred (if possible). E.g.:

...
private const string ALLOWED_MIME_TYPES = "image/png,image/jpeg";
private const string ALLOWED_EXTENSIONS = "png,jpeg";
public IFormFile File { get; set; }
...

public void OnlyPngOrJpeg(){
    var response = File.IsAllowed(ALLOWED_EXTENSIONS, ALLOWED_MIME_TYPES, null, FileCheckLayers.CheckExtensions, FileCheckLayers.CheckExtensions, FileCheckLayers.CheckFileContentTypeToFileExtension, FileCheckLayers.CheckContentType);
    if(!response.IsAllowed) {
      //do whaterver you need to do, e.g.:
      if(response.ErrorType == ErrorType.InvalidExtension)
        throw new Exception("Invalid file, only PNG or JPEG files are allowed.");
      
      //or e.g.:
      throw new Exception(response.ErrorMessage);
    }
}

The BarrierResponse:

    public class BarrierResponse
    {
        public string ErrorMessage { get; set; }
        public bool IsAllowed { get; set; } = true;
        public ErrorType? ErrorType { get; set; }
        
        ...
    }

The ErrorType:

    public enum ErrorType
    {
        InvalidExtension = 0,
        ContentTypeToExtensionMismatch,
        InvalidContentType,
        FileSizeExceeded,
        Unknown
    }

The MimeType mapper is mainly built by @samuelneff: https://github.com/samuelneff/MimeTypeMap.