Does MIME type detection use file contents or just the extension?

Content-based detection is used as the primary method when available. File extension detection is only used as a fallback. This means a .jpg file that is actually a PDF will be reported correctly as application/pdf.

What MIME type is returned when detection fails?

Files that cannot be identified are reported as application/octet-stream. Empty files are reported as application/x-empty. Neither case causes the tool to error — all files in the batch are processed and included in the report.

What file types can I upload for MIME detection?

Supported input types include PDF, XLSX, XLS, CSV, DOCX, TXT, PNG, JPG, JPEG, WEBP, ZIP, JSON, TAR, TGZ, and TAR.GZ. Each file can be up to 50MB; batches can be up to 50 files or 2GB total.

Will the original uploaded files be changed?

No. The tool is read-only — it inspects each file and produces a report. Your original files are returned unmodified.

File MIME Type Detection for Software Developers

When building file upload features or APIs, accepting a file at face value is a security and logic risk — the declared extension may not match actual content. Deliteful's File MIME Type Detector inspects file contents directly and returns a tab-separated report of true MIME types, so you can validate inputs before they hit your pipeline.

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Developers commonly encounter mismatched MIME types when users rename files, when clients send malformed uploads, or when migrating legacy assets between systems. Content-based detection — not just extension sniffing — is the only reliable way to know what a file actually is. This tool uses content-based detection as the primary method, falling back to extension-based detection only when content inspection is inconclusive.

The output is a clean, tab-separated .txt report mapping each filename to its detected MIME type — ready to diff, log, or pipe into a validation script. Files that are empty are reported as 'application/x-empty'; truly unrecognizable files surface as 'application/octet-stream', giving you an explicit signal rather than silent failure. Original files are never modified.

How it works

1
Sign in with Google
Create your free Deliteful account in about 3 clicks — no credit card required.
2
Upload files for inspection
Drag and drop up to 50 files (2GB batch max) including PDFs, ZIPs, images, CSVs, DOCX, and more.
3
Download the MIME report
Receive a tab-separated .txt file mapping every filename to its detected MIME type.

Frequently asked questions

Does MIME type detection use file contents or just the extension?: Content-based detection is used as the primary method when available. File extension detection is only used as a fallback. This means a .jpg file that is actually a PDF will be reported correctly as application/pdf.
What MIME type is returned when detection fails?: Files that cannot be identified are reported as application/octet-stream. Empty files are reported as application/x-empty. Neither case causes the tool to error — all files in the batch are processed and included in the report.
What file types can I upload for MIME detection?: Supported input types include PDF, XLSX, XLS, CSV, DOCX, TXT, PNG, JPG, JPEG, WEBP, ZIP, JSON, TAR, TGZ, and TAR.GZ. Each file can be up to 50MB; batches can be up to 50 files or 2GB total.
Will the original uploaded files be changed?: No. The tool is read-only — it inspects each file and produces a report. Your original files are returned unmodified.

Create your free Deliteful account with Google and validate file MIME types across your entire upload batch in seconds.