Why Some File Types Are Blocked from Sharing

Have you ever tried to share a file via email or a cloud service, only to receive an error message saying the file type isn’t allowed? It’s frustrating when you’re trying to share something completely legitimate, but understanding why these restrictions exist can help you work around them safely.

The Security Rationale

File sharing services block certain file types primarily to protect their users from malware and malicious software. Executable files and scripts can contain harmful code that could compromise computers, steal data, or spread to other systems.

Consider this from the platform’s perspective: they’re responsible for millions of users sharing billions of files. A single compromised executable file could potentially harm thousands of people. Blocking high-risk file types is a straightforward way to prevent the most common attack vectors.

Commonly Blocked File Types

Different services have different policies, but these file types frequently face restrictions:

Executables and Scripts

• .exe, .app, .dmg - Application installers
• .bat, .cmd, .sh - Command scripts
• .msi, .dll - System files

Archives (Sometimes)

• .zip, .rar, .7z - Compressed archives that might contain blocked files

Macro-Enabled Documents

• .xlsm, .docm - Office files with embedded code

Web Files (Occasionally)

• .html, .js - Files that could execute in browsers

The rationale varies by file type. Executable files can directly harm systems. Archives are tricky because they can hide dangerous files inside seemingly innocent packages. Macro-enabled documents became a popular malware delivery method in the 2010s, leading many services to treat them cautiously.

Blocking vs. Scanning

There’s an important distinction between blocking files outright and scanning them for threats:

Blocking means the service refuses to accept certain file types, period. This is a blanket policy based on file extension or MIME type. It’s simple to implement but can catch legitimate files.

Scanning means the service checks file contents for known malware signatures, suspicious patterns, or dangerous code. This is more sophisticated and allows legitimate files through while catching actual threats.

Many modern services use a combination: they scan most files but block the highest-risk types entirely. Some services scan archive contents before accepting them, while others block archives to avoid the computational cost of scanning everything inside.

Why Rules Differ Between Platforms

You might notice that email services tend to be more restrictive than dedicated file sharing platforms. There are several reasons for this:

Email services deal with billions of messages daily, often sent to recipients who haven’t explicitly requested the file. The attack surface is enormous, and users may not expect threats in their inbox. Conservative blocking makes sense here.

File sharing platforms typically require both parties to actively participate in the transfer. Recipients expect to download files and are presumably more cautious. These services can afford to be more permissive while still maintaining security scanning.

Enterprise platforms often have the strictest rules because they’re protecting corporate networks. A single compromised file could affect an entire organization, so IT departments err on the side of caution.

Storage and backup services may be more lenient since files are typically private and not being actively distributed to others. You’re backing up your own data, so the platform focuses more on your storage limits than content restrictions.

Working Around Restrictions Safely

If you have a legitimate file that’s being blocked, here are some options:

Use a specialized service: Some file sharing platforms are designed specifically for developers and technical users who regularly need to share executables, scripts, or other file types that general-purpose services block. Research options that fit your needs and have good security practices.

Archive with password protection: Some services that block certain file types will accept password-protected archives. This prevents automated scanning but allows you to share the file. Share the password through a separate channel for added security.

Rename the extension: This is a workaround where you change .exe to .exe.txt or similar. The recipient renames it back after downloading. This defeats the purpose of security filters, so only do this when sharing with people you trust completely, and warn them about the file type.

Use encrypted file transfer: Some services encrypt files during transfer, which can bypass content-based restrictions. The recipient needs the decryption key to access the file.

Physical transfer: For particularly sensitive or large files, physical media like USB drives or external hard drives might be the most straightforward solution.

A Note on Different Service Approaches

Services take different philosophical approaches to file sharing. Some prioritize maximum compatibility and allow most file types, relying on scanning and user responsibility. Others take a more protective stance, blocking anything that could potentially cause harm.

Stash, for example, focuses on straightforward file sharing without imposing many file type restrictions, allowing you to share the files you need while maintaining the security of encrypted transfer from your device. Different tools exist for different needs, and understanding the tradeoffs helps you choose the right one for each situation.

The Bottom Line

File type restrictions exist for good reasons, primarily to protect users from malware and malicious code. While they can occasionally block legitimate files, they prevent far more harm than inconvenience.

When you encounter these restrictions, take a moment to understand why they exist. If you’re trying to share something legitimate, choose a workaround that maintains security rather than bypassing it entirely. And if you’re receiving a file that required special workarounds to share, approach it with appropriate caution.

The goal isn’t to make file sharing difficult—it’s to make it safer for everyone involved.