Allowed file types validation in Flask

Introduction

We check file types to make sure users upload only safe and expected files. This helps keep the app secure and working well.

When users upload profile pictures and you want only images.

When accepting documents and you want to allow only PDFs or Word files.

When uploading audio or video files and you want to restrict formats.

When preventing harmful files like scripts or executables from being uploaded.

Syntax

Flask

ALLOWED_EXTENSIONS = {'png', 'jpg', 'jpeg', 'gif'}

def allowed_file(filename):
    return '.' in filename and \
           filename.rsplit('.', 1)[1].lower() in ALLOWED_EXTENSIONS

The function checks if the file has an extension and if it is in the allowed list.

Use lower() to make the check case-insensitive.

Examples

This example allows only PDF and DOCX files.

Flask

ALLOWED_EXTENSIONS = {'pdf', 'docx'}

def allowed_file(filename):
    return '.' in filename and filename.rsplit('.', 1)[1].lower() in ALLOWED_EXTENSIONS

This example allows only MP3 and WAV audio files.

Flask

ALLOWED_EXTENSIONS = {'mp3', 'wav'}

def allowed_file(filename):
    return '.' in filename and filename.rsplit('.', 1)[1].lower() in ALLOWED_EXTENSIONS

Sample Program

This Flask app lets users upload files. It checks if the file type is allowed (only images). It shows a message if the file is accepted or rejected.

Flask

from flask import Flask, request, redirect, url_for, render_template_string

app = Flask(__name__)

ALLOWED_EXTENSIONS = {'png', 'jpg', 'jpeg', 'gif'}

def allowed_file(filename):
    return '.' in filename and filename.rsplit('.', 1)[1].lower() in ALLOWED_EXTENSIONS

@app.route('/', methods=['GET', 'POST'])
def upload_file():
    message = ''
    if request.method == 'POST':
        file = request.files.get('file')
        if file and allowed_file(file.filename):
            message = f"File '{file.filename}' is allowed and uploaded successfully."
        else:
            message = "File type not allowed. Please upload an image file."
    return render_template_string('''
        <!doctype html>
        <html lang="en">
        <head>
            <meta charset="utf-8">
            <meta name="viewport" content="width=device-width, initial-scale=1">
            <title>Upload File</title>
        </head>
        <body>
            <h1>Upload an Image File</h1>
            <form method="post" enctype="multipart/form-data">
                <input type="file" name="file" aria-label="Choose file to upload">
                <button type="submit">Upload</button>
            </form>
            <p>{{ message }}</p>
        </body>
        </html>
    ''', message=message)

if __name__ == '__main__':
    app.run(debug=True)

Important Notes

Always check file extensions on the server side, never trust client-side checks alone.

Consider also checking the file content type or scanning files for extra security.

Summary

Allowed file types validation helps keep uploads safe and expected.

Use a set of allowed extensions and check the file's extension before saving.

Combine this with other security checks for best protection.

Practice

1. What is the main purpose of validating allowed file types in a Flask upload feature?

easy

A. To compress files before saving

B. To speed up the file upload process

C. To automatically rename files on upload

D. To ensure only safe and expected file formats are accepted

Solution

1. Step 1: Understand file upload risks

   Uploading files without validation can allow harmful files that may damage the server or application.

2. Step 2: Purpose of allowed file types

   Checking allowed file types restricts uploads to safe, expected formats, reducing security risks.

3. Final Answer:

   To ensure only safe and expected file formats are accepted -> Option D

4. Quick Check:

   Allowed file types = safety [OK]

Hint: Allowed types protect from unsafe uploads [OK]

Common Mistakes:

• Thinking validation speeds upload
• Confusing validation with renaming
• Assuming validation compresses files

2. Which of the following is the correct way to check if a filename has an allowed extension in Flask?

easy

A. if filename.split('.')[-1] in allowed_extensions:

B. if filename.endswith(allowed_extensions):

C. if filename in allowed_extensions:

D. if allowed_extensions.contains(filename):

Solution

1. Step 1: Extract file extension correctly

   Splitting the filename by '.' and taking the last part gets the extension.

2. Step 2: Check extension in allowed set

   Using 'in' checks if the extension is in the allowed_extensions set.

3. Final Answer:

   if filename.split('.')[-1] in allowed_extensions: -> Option A

4. Quick Check:

   Split and check extension [OK]

Hint: Split filename by '.' and check last part [OK]

Common Mistakes:

• Using endswith with a set (wrong syntax)
• Checking whole filename in allowed extensions
• Using non-existent 'contains' method

3. Given this Flask snippet, what will be the output if the uploaded file is named photo.png and allowed_extensions = {'png', 'jpg'}?

filename = 'photo.png'
if filename.split('.')[-1] in allowed_extensions:
    result = 'Allowed'
else:
    result = 'Not Allowed'

medium

A. Allowed

B. Not Allowed

C. Error: split method missing

D. Error: allowed_extensions undefined

Solution

1. Step 1: Extract extension from filename

   Splitting 'photo.png' by '.' gives ['photo', 'png'], last part is 'png'.

2. Step 2: Check if extension is allowed

   'png' is in the set {'png', 'jpg'}, so condition is True and result is 'Allowed'.

3. Final Answer:

   Allowed -> Option A

4. Quick Check:

   Extension 'png' in allowed set [OK]

Hint: Check last part after '.' in allowed set [OK]

Common Mistakes:

• Confusing filename with extension
• Assuming split returns error
• Forgetting to define allowed_extensions

4. Identify the error in this Flask file validation code:

allowed_extensions = {'txt', 'pdf'}
filename = 'report.pdf'
if filename.split('.')[-1] == allowed_extensions:
    print('File allowed')
else:
    print('File not allowed')

medium

A. allowed_extensions should be a list, not a set

B. filename.split('.')[-1] is incorrect syntax

C. Using '==' instead of 'in' to check extension

D. Missing import for split function

Solution

1. Step 1: Understand the comparison

   The code compares a string (extension) to a set using '==', which is always False.

2. Step 2: Correct way to check membership

   Use 'in' operator to check if extension is inside allowed_extensions set.

3. Final Answer:

   Using '==' instead of 'in' to check extension -> Option C

4. Quick Check:

   Use 'in' for membership check [OK]

Hint: Use 'in' to check if extension is allowed [OK]

Common Mistakes:

• Using '==' instead of 'in'
• Thinking split needs import
• Confusing set with list requirement

5. You want to allow only image files with extensions 'png', 'jpg', and 'jpeg' in your Flask app. Which code snippet correctly validates the uploaded file's extension and handles the case where the filename has no extension?

hard

A. allowed = {'png', 'jpg', 'jpeg'}
if filename.endswith(tuple(allowed)):
save_file()
else:
reject_file()

B. allowed = {'png', 'jpg', 'jpeg'}
if '.' in filename and filename.rsplit('.', 1)[1].lower() in allowed:
save_file()
else:
reject_file()

C. allowed = ['png', 'jpg', 'jpeg']
if filename.split('.')[-1] in allowed:
save_file()
else:
reject_file()

D. allowed = {'png', 'jpg', 'jpeg'}
if filename.split('.')[1] in allowed:
save_file()
else:
reject_file()

Solution

1. Step 1: Check for presence of extension

   Using '.' in filename ensures the file has an extension to avoid errors.

2. Step 2: Extract extension safely and check membership

   Using rsplit('.', 1)[1].lower() gets the extension after the last dot, then checks if it's in allowed set.

3. Step 3: Handle saving or rejecting file

   If extension is allowed, call save_file(), else reject_file().

4. Final Answer:

   allowed = {'png', 'jpg', 'jpeg'} if '.' in filename and filename.rsplit('.', 1)[1].lower() in allowed: save_file() else: reject_file() -> Option B

5. Quick Check:

   Check dot, split once from right, then 'in' allowed [OK]

Hint: Check '.' then rsplit once, then 'in' allowed [OK]

Common Mistakes:

• Not checking if filename has '.' before splitting
• Using split without limit causing errors
• Using endswith with tuple but missing case normalization