HTML Guides for accept

The accept attribute provides browsers with a hint about which file types the user should be able to select through the file picker dialog. While browsers may still allow users to select other file types, the attribute helps filter the file picker to show relevant files first, improving the user experience.

The W3C validator reports this error when it encounters tokens in the accept attribute that don’t conform to the expected format. The attribute value is parsed as a set of comma-separated tokens, and each token must be one of the following:

• A valid MIME type such as application/pdf, text/plain, or image/png (the / character separating type and subtype is required).
• A file extension starting with a period, such as .pdf, .docx, or .jpg.
• One of three wildcard MIME types: audio/*, video/*, or image/*.

Common mistakes that trigger this error include using bare file extensions without the leading dot (e.g., pdf instead of .pdf), using arbitrary words that aren’t valid MIME types, or including spaces in a way that creates malformed tokens. The HTML specification (WHATWG) defines strict rules for how these tokens are parsed, and violating them produces a validation error.

Getting this attribute right matters for several reasons. First, a correctly specified accept attribute helps users by pre-filtering the file picker, so they don’t have to hunt through unrelated files. Second, assistive technologies may use this attribute to communicate accepted file types to users. Third, some browsers may silently ignore a malformed accept value entirely, removing the helpful filtering behavior you intended.

To fix the issue, review each token in your accept attribute and ensure it matches one of the three valid formats listed above. If you’re unsure of the correct MIME type for a file format, consult the IANA Media Types registry. When in doubt, dot-prefixed file extensions (like .pdf or .docx) are often simpler and more readable.

Examples

Incorrect: bare words without dots or MIME type format

<input type="file" name="document" accept="doc, docx, pdf">

The tokens doc, docx, and pdf are neither valid MIME types (no /) nor valid file extensions (no leading .), so the validator rejects them.

Incorrect: spaces creating malformed tokens

<input type="file" name="photo" accept="image/png, image/jpeg">

While most browsers handle spaces after commas gracefully, the validator may flag this depending on parsing. It’s safest to avoid spaces or keep them minimal.

Correct: using dot-prefixed file extensions

<input type="file" name="document" accept=".doc,.docx,.pdf">

Correct: using valid MIME types

<input type="file" name="document" accept="application/msword,application/vnd.openxmlformats-officedocument.wordprocessingml.document,application/pdf">

Correct: mixing MIME types, extensions, and wildcards

<input type="file" name="media" accept="image/*,.pdf,video/mp4">

This accepts all image types, PDF files, and MP4 videos. Mixing formats is perfectly valid as long as each token individually conforms to the specification.

Correct: using wildcard MIME types for broad categories

<input type="file" name="photo" accept="image/*">

This allows the user to select any image file, regardless of specific format.