Unexpected Behavior for MIME Type

So far in our tutorial we have used a proprietary file extension as an example to showcase the various options across different utilities/tools for file format identification. The results will be generally consistent across all utilities for the common file extensions, but there can be differences.

In our example files directory we have a .vcf file associated with variant-call-format in bioinformatics/genetics fields. However, .vcf is considered a file format standard for electronic business cards. This results in different results across the tools for the same input file.

Tool Output
file text/plain
mimetype text/vcard
xdg-mime text/vcard
siegfried text/vcard


Siegfried with the PRONOM default signature file correctly identified the format as 'Variant Call Format' but had no associated MIME type. Using the MIME-info signature databases results in the text/vcard.

In cases where there may be erroneous file extensions, it is useful to examine the file contents in addition to the file format. Assume there was mistake in renaming of a file and the coatColor.pheno was named coatColor.png without any change in the file contents.

file which examines the contents of the file before reporting its type results in text/plain.

The mimetype and xdg-mime results in image/png. However, using the --debug flag in mimetype it is evident that the MIME type was extracted based on extension. This behavior can be overridden with the use of -M or --magic-only flag which only considers the contents of the file without accounting for the extensions or globs. The result is similar to file with text/plain MIME type.

Siegfried with the MIME-info database also results in image/png MIME type but includes a warning message, indicating signature error.

warning : 'match on filename only; byte/xml signatures for this format did not match'

Using file without the --mime-type flag reports additional information that could be useful for debugging differences in MIME types.