Let's say I have the following:

image_data = """iVBORw0KGgoAAAANSUhEUgAAAAUAAAAFCAYAAACNbyblAAAAHElEQVQI12P4//8/w38GIAXDIBKE0DHxgljNBAAO9TXL0Y4OHwAAAABJRU5ErkJggg=="""

This is just a dot image (from https://en.wikipedia.org/wiki/Data_URI_scheme). But I do not know if it is image or text etc. Is it possible to understand what it is only having this encoded string? I try it in Python, but it is also general question. So any insight in both is highly welcome.

You can't, at least not without decoding, because the bytes that help identify the filetype are spread across the base64 characters, which don't directly align with whole bytes. Each character encodes 6 bits, which means that for every 4 characters, there are 3 bytes encoded.

Identifying a filetype requires access to those bytes in different block sizes. A JPEG image for example, can be identified from the bytes FF D8 or FF D9, but that's two bytes; the third byte that follows must also be encoded as part of the 4-character block.

What you can do is decode just enough of the base64 string to do your filetype fingerprinting. So you can decode the first 4 characters to get the 3 bytes, and then use the first two to see if the object is a JPEG image. A large number of file formats can be identified from just the first or last series of bytes (a PNG image can be identified by the first 8 bytes, a GIF by the first 6, etc.). Decoding just those bytes from the base64 string is trivial.

Your sample is a PNG image; you can test for image types using the imghdr module:

>>> import imghdr
>>> image_data = """iVBORw0KGgoAAAANSUhEUgAAAAUAAAAFCAYAAACNbyblAAAAHElEQVQI12P4//8/w38GIAXDIBKE0DHxgljNBAAO9TXL0Y4OHwAAAABJRU5ErkJggg=="""
>>> sample = image_data[:44].decode('base64')  # 33 bytes / 3 times 4 is 44 base64 chars
>>> for tf in imghdr.tests:
...     res = tf(sample, None)
...     if res:
...         break
...
>>> print res
png

I only used the first 33 bytes from the base64 data, to echo what the imghdr.what() function will read from the file you pass it (it reads 32 bytes, but that number doesn't divide by 3).

There is an equivalent soundhdr module, and there is also the python-magic project that lets you pass in a number of bytes to determine a file type.