Improve binary file check to better handle PDFs

PDF files can have a very long text header before the first NUL byte

The check now reads the first 2KB rather than 128 bytes, and uses the
built-in index() function rather than looping which is very fast. That
still didn't catch all PDFs, even increasing it to 8000 bytes like git
diff didn't catch all PDFs, so also check for %PDF- signature in file.

The increased number of bytes to check remains even with the %PDF-
signature check because it costs very little (page size is min 4KB),
and will potentially catch other binary files with long text headers.

Author: mmrwoods

autoload/fuzzbox/internal/previewer.vim

@@ -3,6 +3,9 @@ vim9script
 import autoload './popup.vim'
 import './filetype.vim'
 
+const PDF_SIGNATURE = '%PDF-'
+const PDF_SIGNATURE_BLOB = list2blob(str2list(PDF_SIGNATURE))
+
 def IsBinary(path: string): bool
     # NUL byte check for binary files, used to avoid showing preview
     # Assumes a file encoding that does not allow NUL bytes, so will
@@ -11,18 +14,27 @@ def IsBinary(path: string): bool
     if !has('patch-9.0.0810')
         # Workaround for earlier versions of Vim with limited readblob()
         # Option to read only part of file finalised in patch 9.0.0810
-        return match(readfile(path, '', 10), '\%x00') != -1
+        var lines = readfile(path, '', 10)
+        if !empty(lines) && type(lines[0]) == v:t_string
+             if lines[0][0 : len(PDF_SIGNATURE) - 1] == PDF_SIGNATURE
+                 return true
+             endif
+        endif
+        return match(lines, '\%x00') != -1
     endif
     return IsBinaryBlob(path)
 enddef
 
 # Note: use of legacy function a workaround for compilation failing when
 # readblob() would be called with invalid args on earlier Vim versions
+# Check first 2048 bytes as some binary formats have a long text header
+# Specific check for PDF file signature because PDF headers can be huge
 function IsBinaryBlob(path)
-    for byte in readblob(a:path, 0, 128)
-        if byte == 0 | return v:true | endif
-    endfor
-    return v:false
+    let blob = readblob(a:path, 0, 2048)
+    if blob[0 : len(s:PDF_SIGNATURE) - 1] == s:PDF_SIGNATURE_BLOB
+        return v:true
+    endif
+    return index(blob, 0) != -1
 endfunction
 
 def FTDetectModelines(content: list<string>): string