Verify string exists as key or value in Python dictionary?

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dictionary

Python3.6

I'm building a scraper/crawler for linux directories. in essence the program will take users input for startpoint (EX: /home/user/Pictures/) and endpoint (EX: /home/user/Pictures/) as well as a file type to scrape for (which is where my question comes in)
I'm storing acceptable file extension types in a dictionary w/ nested lists like so:

file_types = {'audio': ['mp3', 'mpa'], 'images': ['png', 'jpg']}

if I store the users input as the variable scrape_for how can I validate that the string in the variable scrape_for exists in the dictionary file_types?

What I have tried:

this is my current block of code which does the following:
1. take user input for start point
2. verify startpoint is a valid directory
3. take user input for end point
4. validate end point is both a valid directory and sub directory of start point
5. print options of file extensions for user to choose from

import os
ftypes = {'audio': ['mp3', 'mpa', 'wpi', 'wav', 'wpi'], 'images': ['png', 'jpg', 'jpeg', 'gif', 'bmp'], 'text': ['txt', 'doc', 'pdf'], 'video': ['mp4', 'avi', '3g2', '3gp', 'mkv', 'm4v', 'mov', 'mpg', 'wmv', 'flv'], 'executable': ['apk', 'bat', 'bin', 'exe', 'py', 'wsf', 'com', 'cgi', 'pl']}

def UserInput():
#User inputs Start Point
    Spoint = input('Where to start: \n')
#check validity of input
    if os.path.isdir(Spoint):
        print('Scraping will begin at: ' + Spoint)
    elif not os.path.isdir(Spoint):
        print('Not a valid directory')
        exit()
#User input for End Point
    Epoint = input('\n\nWhat directory would you like to stop scraping at? \n')
# Check if Endpoint is a valid SubDirectory of the parent directory
    if os.path.isdir(Epoint) and len(Epoint) >= len(Spoint):
        print('\n\nScraping will end at: ' + Epoint)
    elif os.path.isdir(Epoint) or len(Epoint) >= len(Spoint):
        print('Error w/ End Point directory, make sure directory is formatted correly, and is a sub directory of your Starting Point')
        exit()

#User input for filetype
    for k,v in ftypes.items():
        print(k, v)
    ScrapeType = input('Please enter The extension youd like scraped: \n')