Is there a built in way in Ruby/Rails to detect the mime type of a file given its URL, and not relying on its extension?
For example lets say there is an image file at an external URL that I want to get the mime type of: https://www.example.com/images/file
The file does not have an extension, but let's assume it is a jpeg.
How can I verify this/get the file's mime type in Rails? Would ideally love a way to do this with built in functionality and not have to rely on a third party gem.
I don't think it's worth not using a third party gem for this. The problem space is well documented and stable, and most of the libraries are too.
But if you must, it can be done without an external gem. Especially if you're going to constrain yourself to a small subset of file types to "whitelist". The "magic number" pattern for most image files is pretty straightforward once you get the file on your disk:
image = File.new("filename.jpg","r")
irb(main):006:0> image.read(10)
=> "\xFF\xD8\xFF\xE0\x00\x10JFIF"
Marcel, which you linked in your reference, if nothing else, can be a great reference for the magic number sequences you'll need:
MAGIC = [
['image/jpeg', [[0, "\377\330\377"]]],
['image/png', [[0, "\211PNG\r\n\032\n"]]],
['image/gif', [[0, 'GIF87a'], [0, 'GIF89a']]],
['image/tiff', [[0, "MM\000*"], [0, "II*\000"], [0, "MM\000+"]]],
['image/bmp', [[0, 'BM', [[26, "\001\000", [[28, "\000\000"], [28, "\001\000"], [28, "\004\000"], [28, "\b\000"], [28, "\020\000"], [28, "\030\000"], [28, " \000"]]]]]]],
# .....
]