(rails) how to validate whether an uploaded .txt file is not, say, an image file?

I have a upload text file field, and with it I plan to save the file somewhere and then store the location of the file in a database. However, I want to make sure the file they uploaded is a .txt file, and not, say, an imag开发者_如何学Ce file. I imagine this happens in the validation step. How does one validate such a thing? Also, how do you get the filename of the uploaded file? I could always just check if it said '.txt' but for future reference knowing how to validate without just the filename would be helpful.

Trying to validate the contents of a file based on the filename extension is opening the door for major hackerdom. It's trivial to change the extension and upload the file.

If you are on a Mac/Linux/Unix-based system the OS "file" command is the standard because it looks inside the file for key bytes that flag file types. http://en.wikipedia.org/wiki/File_(Unix) I'm not sure what's available for Windows, but this might help: Determine file type in Ruby

One way of doing it, the simple way really, would be to pass the file through an image loader, preferably one that handles multiple common formats, and see if it throws an error.

The other way is to manually check the file header for common image format headers. For example, .bmp files start with BM. Other formats have their own specific markings you can use.