How do i replace text before a certain point in a string?

There are multiple ways to do this, but the simplest way is probably to use rsplit.

filename = "somefile.gif"
basename, ext = filename.rsplit(".", maxsplit=1)
replacement = "image/" + ext

you can use the split function in python for the strings to get the extension, the split function is a property of standard strings in python which takes an argument as the separator and divides the string into parts. In your case extensions are signed via a ., so you can divide the string into several parts and take the last one.

So the code should be,

def main():
    ext = ["gif", "jpeg", "jpg", "png", "pdf", "txt", "zip"]
    file = input("What file would you like to run? \n>> ")
    extension = file.split('.')[-1] # here split returns a list of divisions, the extension is the very last part of that list
    if extension in ext:
        print("images/" + extension)
    else:
        print("File extension not valid.")

main()

I am not sure what you mean by adding images/ with the extension but however you can now edit the code suitable to your program

For your specific case, you mentioned you only want to replace the start of the file. In this case, try:

def main():
    ext = (".gif", ".jpeg", ".jpg", ".png", ".pdf", ".txt", ".zip")
    file = input("What file would you like to run? \n>> ")
    for possible_end in ext:
        if file.endswith(possible_end):
            print("image" + possible_end)

main()

You need to loop through all the possible endings in the tuple, not just provide the tuple itself. This'll print 'image' + the appropriate file end. Otherwise, it won't print anything.

you could work with a sed style regular expression...

's|^.*\.([a-z]{3,4})$|image/\1|'

eg, in bash...

echo '
something.jpg
thingy.png
junk.tiff
' | sed -E 's|^.*\.([a-z]{3,4})$|image/\1|'

image/jpg
image/png
image/tiff

This captures and saves the extension with (\.[a-z]{3,4}$) (ie, whatever text is at the end of the string that has a dot followed by 3 or 4 lower case letters) then pastes the captured text (with \1) to the end of the string 'image', replacing the original basename (minus extension), which is matched by ^.*. This expression can be made case insensitive if required.

If accepted extensions need to be limited:

echo '
something.jpg
thingy.png
junk.tiff
' | sed -E 's:^.*\.(jpg|png|tiff)$:image/\1:'

image/jpg
image/png
image/tiff

This syntax (or similar) likely works in some python library like re.

[Edited to replace dot with slash...]

Using a regular expression you can both validate and isolate the components of the input string that you're interested in as follows:

import re

def main() -> str:
    ex = r"^(.+)\.(gif|jpeg|jpg|png|pdf|txt|zip)$"
    file = input("What file would you like to run? ")
    return f"image/{m.group(1)}" if (m := re.match(ex, file)) else ""

print(main())

Example:

What file would you like to run? abc.gif
image/abc