Allow only specific file type to upload in blazor

I have a wrapper to Steve Sanderson's InputFile that has an AllowedExtensions property. This is an after the fact filter, meaning the user has to upload the file and then you tell them that file extension isn't allowed. There are entire threads on doing the pre upload way, and it is difficult to say the least.

This is the way I did it after the upload:

Nuget: DataJuggler.Blazor.FileUpload

The full source code including a Blazor sample project is located here:

https://github.com/DataJuggler/BlazorFileUpload

The most relevant part is summarized here:

using DataJuggler.Blazor.FileUpload

<FileUpload CustomSuccessMessage="Your file uploaded successfully." 
    OnReset="OnReset" ResetButtonClassName="localbutton" ShowStatus="false"
    PartialGuidLength="10" MaxFileSize=@UploadLimit FilterByExtension="true" 
    ShowCustomButton="true"  ButtonText="Start" OnChange="OnFileUploaded"
    CustomButtonClassName="startbutton" ShowResetButton="false" 
    AppendPartialGuid="true" AllowedExtensions=".jpg;.png;"
    CustomExtensionMessage="Only .jpg and .png files are allowed." 
    InputFileClassName="customfileupload" Visible=false
    FileTooLargeMessage=@FileTooLargeMessage>
</FileUpload>

Notice the two properties for AllowedExtensions, and CustomExtensionMessage.

Here is the relevant part of code where I handle the FileUploaded:

// Create a new instance of a 'FileInfo' object.
FileInfo fileInfo = new FileInfo(file.Name);

// I don't know if it's even possible for an extension to be upper case
uploadedFileInfo.Extension = fileInfo.Extension.ToLower();

// if FilterByExtension is true and the AllowedExtensions text exists
if ((FilterByExtension) && (!String.IsNullOrWhiteSpace(AllowedExtensions)))
{
    // verify the extension exists
    if (!String.IsNullOrWhiteSpace(fileInfo.Extension))
    {
        // If the allowed extensions // fixed issue where uploading 
        abort = !AllowedExtensions.ToLower().Contains(fileInfo.Extension.ToLower());
    }
    else
    {
        // you must have an extension
        abort = true;
    }
}

Maybe this will give you some ideas.

If you want to see a live site that uses it, I just published this a couple of days ago: https://pixeldatabase.net - Edit Images with BQL - Bitmap Query Language