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FileUpload and File type

Lilian Chase
Greenhorn

Joined: Jun 14, 2010
Posts: 24
I wrote a servlet to do file upload using the commons jar. This part is working fine and I'm able to upload files successfully to the server.

My question is How do I determine the file type of each file? Users can upload files with any extensions as long as the content is text.

Eg,

The user can upload test1.txt, test2.abc, test3.fa, if all 3 files are text-based.
On the other hand, I want to stop the upload if the file is an image file, even if the filename is image.txt.


So, is there a way to check and validate the content of the file and determining the file type without restricting the file extensions?
Greg Charles
Sheriff

Joined: Oct 01, 2001
Posts: 2853
    
  11

It's sometimes possible to tell the type of a file based on its content. One way is to check the magic number, though not all file types have one, and there's no guarantee there won't be conflicts (though it's statistically improbable). For example, GIF files all start with the ASCII string GIF87a or GIF89a. A text file or pure binary file might also begin with this combination, but it's unlikely. Wikipedia will give you a place to start.
Rob Spoor
Sheriff

Joined: Oct 27, 2005
Posts: 19718
    
  20

Except if it's a text file discussing the GIF89a format


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