Sniff a type from bytes

Runs the WHATWG MIME Sniffing Standard's "MIME type sniffing algorithm" over the first bytes of a raw vector, with its three inputs: the supplied type (a Content-Type value, or NULL), the no-sniff flag, and the sniffing context.

Usage

mime_sniff(
  x,
  type = NULL,
  no_sniff = FALSE,
  context = c("none", "browsing", "image", "audio_video", "font"),
  limit = 4096L
)

Arguments

x

A raw vector, or a character vector of paths, which are read as mime_type() reads them; an unreadable path gives a row with type NA.

type

The supplied type, such as a Content-Type header value, or NULL when there is none. A value that does not parse as a MIME type is treated as NULL.

no_sniff

TRUE for the no-sniff flag (X-Content-Type-Options: nosniff): a supplied type is then returned as is.

context

One of "none", "browsing", "image", "audio_video" or "font".

limit

The number of bytes to read; 0 reads them all. A raw vector longer than limit is sniffed on its first limit bytes.

Value

A zumime_result data frame with one row per path (one for a raw vector) and columns path (NA for a raw vector), type (a canonical type), confidence (an ordered factor: none < extension < probable < definite), charset, inner, encrypted, complete, newline ("lf", "crlf", "cr", "mixed", or NA) and interpreter. inner and complete are always NA here; mime_type() fills them.

Details

The "browsing" context is what a browser computes for a navigation: the standard's tables and nothing else. The default "none" asks the plain file question: the same algorithm, then zumime's own signatures (compression wrappers, tar, ISO 9660, RAR 5, 7z, fonts, WebAssembly) where the standard found only binary bytes. "image", "audio_video" and "font" are the standard's context-specific algorithms: they look only for types of that kind and otherwise return the supplied type.

A text result carries charset, from the first rule that answers: the supplied type's charset parameter, a byte-order mark, an XML declaration, an HTML <meta> prescan, valid UTF-8, then the C1 rule. The C1 rule gives windows-1252 when a byte in 0x80 to 0x9F appears, else ISO-8859-1. It is a guess, so it lowers confidence to "probable".

References

https://mimesniff.spec.whatwg.org/

See also

mime_parse() for the supplied type's grammar.

Examples

png <- as.raw(c(0x89, 0x50, 0x4e, 0x47, 0x0d, 0x0a, 0x1a, 0x0a))
mime_sniff(png)
#> <zumime_result: 1 row>
#>   path      type confidence charset inner encrypted complete newline
#> 1 <NA> image/png   definite    <NA>  <NA>        NA       NA    <NA>
#>   interpreter
#> 1        <NA>
mime_sniff(charToRaw("<!DOCTYPE html><title>x</title>"))
#> <zumime_result: 1 row>
#>   path      type confidence charset inner encrypted complete newline
#> 1 <NA> text/html   definite   UTF-8  <NA>        NA       NA    <NA>
#>   interpreter
#> 1        <NA>

# A server that says text/plain about a binary file
mime_sniff(png, type = "text/plain")
#> <zumime_result: 1 row>
#>   path                     type confidence charset inner encrypted complete
#> 1 <NA> application/octet-stream       none    <NA>  <NA>        NA       NA
#>   newline interpreter
#> 1    <NA>        <NA>

# The supplied type wins under nosniff
mime_sniff(png, type = "text/csv", no_sniff = TRUE)
#> <zumime_result: 1 row>
#>   path     type confidence charset inner encrypted complete newline interpreter
#> 1 <NA> text/csv       none    <NA>  <NA>        NA       NA    <NA>        <NA>