Web forms and upload portals often reject filenames due to specific formatting rules. These rules typically prohibit characters that could pose security risks (like <, >, :, /, \, |, ?, *), limit filename length, disallow certain prefixes or suffixes, or ban spaces entirely. Different systems enforce varying strictness, sometimes even blocking spaces or non-English characters depending on server configuration. The goal is to prevent malicious scripts from hiding in names, ensure compatibility across operating systems, and guarantee the file path is valid.

For instance, a file named report_2024/03.docx might be rejected because the forward slash / is often an illegal character. Similarly, an image file uploaded from a Mac called vacation photo 1.jpg could fail on a portal requiring underscores instead of spaces to avoid URL encoding issues like %20 breaks in links. Content Management Systems (CMS), cloud storage services, and application portals commonly apply these restrictions.

These rules enhance security by preventing path traversal attacks and script injection. However, strict limitations can frustrate users with valid filenames and represent design challenges. Upload portals should ideally provide clear error messages listing acceptable characters to improve user experience while maintaining necessary security protocols.