How to Check If a File Is a Valid Image File

How to check if a file is a valid image file?

A lot of times the first couple chars will be a magic number for various file formats. You could check for this in addition to your exception checking above.

How to check if a file is a valid image?

What is usually done is checking if the file has the right magic number for the image file format you want. While this test is not super accurate, it is usually good enough. You can use code like this:

package foo

import "strings"

// image formats and magic numbers
var magicTable = map[string]string{
    "\xff\xd8\xff":      "image/jpeg",
    "\x89PNG\r\n\x1a\n": "image/png",
    "GIF87a":            "image/gif",
    "GIF89a":            "image/gif",
}

// mimeFromIncipit returns the mime type of an image file from its first few
// bytes or the empty string if the file does not look like a known file type
func mimeFromIncipit(incipit []byte) string {
    incipitStr := []byte(incipit)
    for magic, mime := range magicTable {
        if strings.HasPrefix(incipitStr, magic) {
            return mime
        }
    }

    return ""
}

determine if file is an image

Check the file for a known header. (Info from link also mentioned in this answer)

The first eight bytes of a PNG file always contain the following (decimal) values: 137 80 78 71 13 10 26 10

file upload: check if valid image

Firstly add accept="image/*" on your input, to accept only image files

<input type="file" name="uploadPicture" accept="image/*" onChange="validateAndUpload(this);"/>

Second, you can create image object to see if it is true image

function validateAndUpload(input){
    var URL = window.URL || window.webkitURL;
    var file = input.files[0];

    if (file) {
        var image = new Image();

        image.onload = function() {
            if (this.width) {
                 console.log('Image has width, I think it is real image');
                 //TODO: upload to backend
            }
        };

        image.src = URL.createObjectURL(file);
    }
};​

How to check if image file is valid?

First of all, don't try to use System.Drawing in .NET Core applications. It's deprecated and works only on Windows anyway. The MSDN docs themselves suggest using ImageSharp or SkiaSharp instead.

Image files start with bytes that identify the file format. You'll have to read at least some of the file's contents to read image metadata like the image size, resolution etc. You can use ImageSharp's Identify method to read only the format and image properties, without loading the entire image.

You can read an uploaded file's contents using IFormFile.OpenReadStream.

using var stream=image.OpenReadStream();
try
{
    var imageInfo=Image.Identify(stream, out var format);
    if(imageInfo!=null)
    {
        var formatName=format.Name;
        var width=imageInfo.Width;
        var height=imageInfo.Height;
    }
}
catch(InvalidImageContentException exc)
{
   //Invalid content ?
}

The format parameter is an IImageFormat value that contains information about the image format, including its name and mime types.

The IImageInfo object returned contains the image dimensions, pixel type, resolution etc.

The method documentation explains that the return value will be null if no suitable decoder is found:

The IImageInfo or null if a suitable info detector is not found.

But an exception will be thrown if the content is invalid:

InvalidImageContentException Image contains invalid content.

Without testing this, I assume that a text file will result in a null but a file with just a GIF header without valid content will result in an exception.

You can use ImageSharp to resize the image or convert it to another format. In that case it's not enough to just load the metadata. You can use Load to load the image from the stream, detect its format and then manipulate it.

using var stream=image.OpenReadStream();
var image=Image.Load(stream, out var format);

var formatName=format.Name;
if (notOk(formatName,image.Height,image.Width))
{
    using var outStream=new MemoryStream();
    image.Mutate(x => x.Resize(desiredWidth, desiredHeight));
    image.SaveAsPng(outStream);
    outStream.Position=0;
    //Store the contents of `outStream`
}

Test if a file is an image file

This works pretty well for me. Hope I could help

import javax.activation.MimetypesFileTypeMap;
import java.io.File;
class Untitled {
    public static void main(String[] args) {
        String filepath = "/the/file/path/image.jpg";
        File f = new File(filepath);
        String mimetype= new MimetypesFileTypeMap().getContentType(f);
        String type = mimetype.split("/")[0];
        if(type.equals("image"))
            System.out.println("It's an image");
        else 
            System.out.println("It's NOT an image");
    }
}

Determine whether a file is a valid image format

Use

Image.FromFile(path); 

It throws if the file of path is not a valid image.

How to check that an uploaded file is a valid Image in Django

Good news, you don't need to do this:

class ImageField(upload_to=None, height_field=None, width_field=None, max_length=100, **options)

Inherits all attributes and methods from FileField, but also validates that the uploaded object is a valid image.

https://docs.djangoproject.com/en/1.10/ref/models/fields/#django.db.models.ImageField

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