Question

Using a Java servlet, is it possible to detect the true file type of a file, regardless of its extension?

Scenario: You only allow plain text file uploads (.txt and .csv) The user takes the file, mypicture.jpg, renames it to mypicture.txt and proceeds to upload the file. Your servlet expects only text files and blows up trying to read the jpg.

Obviously this is user error, but is there a way to detect that its not plain text and not proceed?

Answer 1

You can do this using the builtin URLConnection#guessContentTypeFromStream() API. It’s however pretty limited in content types it can detect, you can then better use a 3rd party library like jMimeMagic.

See also:

• Best way to determine file type in Java
• When do browsers send application/octet-stream as Content-Type?