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How to Upload Files on a Server Using RESTful API?

Illustration showing a laptop and mobile device connected by REST API to upload files, depicting the interactivity and code elements involved in the API integration process.

The world of web apps relies on data exchange. RESTful APIs (Application Programming Interfaces), aka. REST APIs give programs a systematic mechanism to communicate and share data. One critical feature an API provides is the ability to upload files to a server. This blog serves as your guide to understanding and implementing REST API to upload files.

What is RESTful API?

RESTful or REST API file is an application programming interface that is used to exchange information securely over the internet between computers and servers. This information can be shared through various formats such as Python, JSON, HTTP, HTML, XLT, PHP, or plain text, with JSON being the most popular.

Multipart encoding

Another concept to understand is multipart encoding. When a file is uploaded through a web form, the data is not sent as a single unit. Instead, it’s broken down into parts using multipart encoding. One part represents the form data (e.g., additional information besides the file), and the other part carries the actual file content.

Upload limits

Before getting into the code, it’s essential to consider some practical aspects. File size limitations on the server side can prevent uploading excessively large files. Additionally, implementing checks for allowed file types (e.g., images and documents) helps ensure the server receives what it’s designed to handle.

How do I use the REST API to upload files?

Building the client-side

  • Create a form: Build an HTML form with a file input element (<input type=”file”>). Construct the form with enctype as multipart/form-data to allow file uploads.

  • Handle user selection: Utilize JavaScript to capture the file selected by the user.

  • Send the request: Send the FormData object containing the file to the server without setting the Content-Type header manually, as FormData takes care of it.

Building the server-side

To create a server-side snippet for handling file uploads using a REST API, you would typically use a backend framework like Express for Node.js with a middleware like multer for handling multipart/form-data. Below is an example using Express and Multer:

This code sets up a basic Express server that listens for POST requests at the /upload endpoint, receives a file uploaded via the file form input, saves it to an ‘uploads/’ directory on the server, and then responds with a success message.

Remember to install the necessary Node.js packages (express and multer) before running this code. Also, adapt the endpoint and storage configuration according to your specific requirements.

  • Define the endpoint: First, Create an API endpoint (URL) that handles file uploads. This endpoint should only accept POST requests. The Express.js app sets up a POST endpoint at /upload to handle file uploads.
  • Access uploaded file: On the server side, retrieve the uploaded file data from the request object. This often involves accessing the file object within a dictionary based on the file input name in the form. This example uses multer middleware to access the uploaded file, represented as req.file in the request handler.
  • Validate and process: Implement checks on the uploaded file, such as verifying its type or size. You can also perform any necessary processing on the file content here. While the example does not show explicit validation, you can add this within the route handler by checking req.file.
  • Save the file: Store the uploaded file securely on the server. However, this might involve generating a unique filename and saving it in a designated directory. Multer’s storage configuration saves the file with a unique filename in the ‘uploads/’ directory.
  • Respond to the client: Send a response back to the client indicating success or failure (with appropriate error messages). You might also include additional information, like the saved filename. In this example the server sends a JSON response back to the client indicating a successful upload.
  • For specific validations like file type or size, you’ll need to expand the route handler logic to include those checks before saving the file.

Security considerations

It’s crucial to consider security when implementing REST API to upload files. Here are some essential points to remember:

  • File Type Validation: Enforce checks on the server side to ensure only allowed file types are uploaded. This can prevent potential security vulnerabilities or unexpected behavior.
  • Input Sanitization: Sanitize any user-provided data associated with the upload, such as filenames, to prevent malicious code injection attempts.
  • Access Control: Additionally, Implement access control measures to restrict who can upload files to your server. This can involve authentication and authorization checks.

Testing the file upload functionality

Introduction to Postman

Source: Postman

Postman is a software platform that is specially built to interact with API files (application programming interfaces). It serves as a full toolset for developers to quickly construct, test, debug, and document APIs. You can use it to check if your REST API to upload files is functioning properly. Here is a rundown of Postman’s main functionalities:

  • Sending Requests: Postman allows you to construct and send HTTP requests (GET, POST, PUT, etc.) to API endpoints. You can easily specify the URL, headers, body parameters, and other request details.
  • Building Collections: Organize your API interactions by grouping related requests into collections. This helps streamline testing workflows and manage different API versions.
  • Testing Responses: Postman displays the server’s response to your requests, including status codes, headers, and response body. You can validate responses against expected data formats using assertions.
  • Environment Management: Simulate different development environments (e.g., development, staging, production) by managing environment-specific variables and configurations within Postman.

How to use Postman?

Rest API to Upload Files

Tools like Postman are excellent for testing API requests. Here’s how to use Postman to test the file upload functionality:

  1. Create a POST file Request: In Postman, create a new request and set the HTTP method to POST.
  2. Set the URL: Enter the URL of your server-side API endpoint (e.g., http://localhost:5000/upload).
  3. Add the File: In the “Body” tab of Postman, select the “form-data” option. Click on “file” and choose the file you want to upload.
  4. Send the Request: Click the “Send” button in Postman software to send the POST request with the file data.

Additionally, Postman will display the server’s response. A successful upload should result in a status code of 200 (OK) along with the success message and potentially the unique filename assigned to the uploaded file (depending on your server-side implementation).

Why Should You Choose REST API to Upload Files?

REST APIs offer several advantages over other methods for uploading files to a server, making them a popular choice for web applications:

Standardized Approach

REST APIs adhere to well-defined architectural principles, ensuring consistent communication between applications. This simplifies development and integration compared to custom protocols.

Flexibility and Scalability

REST APIs can handle various data types, including files, making them highly adaptable to different use cases. They also scale well to accommodate increasing file upload API volumes as your application grows.

Easy Integration

The use of standard HTTP methods (POST for uploads) and data formats (often JSON or MultiPart form data) allows for seamless integration with various client-side technologies like Javascript and HTML forms.

Security Potential

REST APIs provide a framework for implementing robust security measures like access control and authentication. This ensures that only an authorized user can upload file to API and protect your server from unauthorized access.

Resource Management

REST APIs enable efficient resource management on the server. By separating the file upload API process into a dedicated endpoint, you can optimize server resources for handling uploads without affecting other functionalities.


RESTful design principles promote clean code separation and well-defined interactions. This makes REST API code easier for developers to maintain and understand.

Tooling and Ecosystem

A vast ecosystem of tools and libraries exists to support REST API development and testing. Tools like Postman and frameworks like Flask or Django offer functionalities specifically designed for handling REST API to upload files.


This blog has equipped you with the knowledge and steps to implement file upload functionality in your RESTful APIs. Remember to tailor the code examples to your specific programming language and framework. 

By following these guidelines and considering security aspects, you can develop a robust REST API to upload files that seamlessly handle file uploads. 

Also, check APILayer’s marketplace for more functional APIs for your project or sell your own API services to reach a wider audience.


What are some common challenges faced when using REST API to upload files?

There are a few challenges to consider:

  • Large file uploads: Uploading very large files can take time and consume resources. You might need to implement progress bars or chunking mechanisms for efficient handling.
  • Security vulnerabilities: Malicious users might try to upload harmful files. Implementing strict validation and access control is crucial.
  • Error handling: Uploads can fail due to various reasons like network issues or exceeding file size limits. Proper blunder handling with informative messages is essential for a good user experience.

Can I upload multiple files to the server once using this approach?

The approach presented focuses on uploading a single file. However, some frameworks and libraries offer functionalities for handling multiple file uploads. These often involve sending each file as a separate part within the MultiPart form data.

What are some security best practices for file uploads?

Here are some key practices:

  • Validate file types: Only allow uploads of specific file types (e.g., images, documents) based on your application’s needs.
  • Sanitize user input: Clean any user-provided data associated with the upload, such as filenames, to prevent potential code injection attacks.
  • Access control: Implement mechanisms to restrict who can a upload file to API. This can involve authentication and authorization checks on the server side.
  • Store uploads securely: Consider using secure storage mechanisms for uploaded files, depending on their sensitivity.

How to upload large files (1GB+) through a RestAPI?

Typically, file size limitations are enforced on the server side. You can configure these limits based on your server’s capacity and application requirements and allow the uploading of large files. It’s also good practice to inform users about the maximum allowed file size before they attempt an upload.

What tools can I use besides Postman to test file uploads?

Aside from Postman, there are numerous tools available for testing REST API to upload files requests, including:

  • Curl: A command-line utility for sending HTTP requests.
  • Fiddler: A web debugging proxy that can observe and manipulate HTTP traffic.
  • SoapUI: An open-source tool for testing online services, such as RESTful APIs.

These tools provide varying functionality and levels of sophistication. Select the one that best fits your development environment and interests.

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