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Overview

Nuclei allows modelling templates that can match/extract on the local file system. 
# Start of file template block
file:

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Extensions

To match on all extensions (except the ones in default denylist), use the following - 
extensions:
  - all
You can also provide a list of custom extensions that should be matched upon. 
extensions:
  - py
  - go
A denylist of extensions can also be provided. Files with these extensions will not be processed by nuclei. 
extensions:
  - all

denylist:
  - go
  - py
  - txt
By default, certain extensions are excluded in nuclei file module. A list of these is provided below- 
3g2,3gp,7z,apk,arj,avi,axd,bmp,css,csv,deb,dll,doc,drv,eot,exe,
flv,gif,gifv,gz,h264,ico,iso,jar,jpeg,jpg,lock,m4a,m4v,map,mkv,
mov,mp3,mp4,mpeg,mpg,msi,ogg,ogm,ogv,otf,pdf,pkg,png,ppt,psd,rar,
rm,rpm,svg,swf,sys,tar,tar.gz,tif,tiff,ttf,txt,vob,wav,webm,wmv,
woff,woff2,xcf,xls,xlsx,zip

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More Options

max-size parameter can be provided which limits the maximum size (in bytes) of files read by nuclei engine. As default the max-size value is 5 MB (5242880), Files larger than the max-size will not be processed.
no-recursive option disables recursive walking of directories / globs while input is being processed for file module of nuclei.

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Matchers / Extractors

File protocol supports 2 types of Matchers -
Matcher TypePart Matched
wordall
regexall
Extractors TypePart Matched
wordall
regexall

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Example File Template

The final example template file for a Private Key detection is provided below. 
id: google-api-key

info:
  name: Google API Key
  author: pdteam
  severity: info

file:
  - extensions:
      - all
      - txt

    extractors:
      - type: regex
        name: google-api-key
        regex:
          - "AIza[0-9A-Za-z\\-_]{35}"

# Running file template on http-response/ directory
nuclei -t file.yaml -target http-response/

# Running file template on output.txt
nuclei -t file.yaml -target output.txt
More complete examples are provided here