Universal Device Independant Bitmap (DIB) to BMP Converter

dib2bmp.py

#!/usr/bin/env python3
'''
          DO WHAT THE FUCK YOU WANT TO PUBLIC LICENSE
TERMS AND CONDITIONS FOR COPYING, DISTRIBUTION AND MODIFICATION

    0. You just DO WHAT THE FUCK YOU WANT TO.

Universal DIB to BMP converter. <d$pid>
'''
import sys
import math

from binascii import hexlify

def usage():
    print("python {} <path to DIB file> <output file path>".format(sys.argv[0]))
    sys.exit(0)

def conv(size):
    return ''.join(map(chr, [size & 0xFF, (size >> 8) & 0xFF, (size >> 16) & 0xFF, (size >> 24) & 0xFF]))

def length():
    try:
        with open(sys.argv[1], 'rb') as fd:
            length = len(fd.read())
            fd.close()
            return length
    except Exception:
        raise Exception

def prepend(data, path):
    try:
        with open(sys.argv[1], 'rb') as fd:
            original = fd.read()
            fd.close()
        with open(path, 'wb') as fd:
            new = data + original
            fd.write(new)
            fd.close()
    except Exception:
        raise Exception

if __name__ == '__main__':
    if len(sys.argv) <= 2:
        usage()
    
    offset = int(54 + math.pow(2, 1) * 4)
          
    # Header Generation
    data  = 'BM'.encode()                         # ID Field
    data += conv(offset + length()).encode()      # BMP Size
    data += conv(0).encode()                      # Unused
    data += conv(offset).encode()                 # Pixel Data Offset

    print("[DIB TOOL] Crafting header...")

    print("[DIB TOOL] Header crafted for file: {}\n\n\t{}\n".format(
        sys.argv[1], hexlify(data)))

    print("[DIB TOOL] Prepending header to new file: {}".format(sys.argv[2]))
          
    # Prepend Crafted Header to File
    prepend(data, sys.argv[2])

    print("[DIB TOOL] Done")

This doesn't seem to work. I tried it on some files I was able to able to convert with an online convertor. But I'd love to do it in Python. Any help would be appreciated. Unfortunately github won't let me share the DIB file here.