WannaCry

There are a number of files and folders wannacrypt will avoid.
Some because it's entirely pointless and others because it might destabilize the system.
During scans, it will search the path for the following strings and skip over if present.

    "Content.IE5"
    "Temporary Internet Files"
    " This folder protects against ransomware. Modifying it will reduce protection"
    "\Local Settings\Temp"
    "\AppData\Local\Temp"
    "\Program Files (x86)"
    "\Program Files"
    "\WINDOWS"
    "\ProgramData"
    "\Intel"
    "$\"

The strange looking folder name referring to ransomware was probably used during tests
or perhaps the authors left it behind by accident.

Naturally, it will avoid encrypting itself and skips the following.

    @Please_Read_Me@.txt
    @WanaDecryptor@.exe.lnk
    @WanaDecryptor@.bmp

It will also skip files that have extensions: .DLL, .EXE, .WNCRY and .WNCRYT

For everything else, there are 2 tables with file extensions it checks,
the first and default list consists of

  .doc, .docx, .xls, .xlsx, .ppt, .pptx, .pst, .ost, .msg,
  .eml, .vsd, .vsdx, .txt, .csv, .rtf, .123, .wks, .wk1, .pdf,
  .dwg, .onetoc2, .snt, .jpeg, .jpg

The other is a much more extensive list, but is not used by the sample
I have.

It's not clear what the purpose of multiple file extensions are, but perhaps
the ransomeware uses a library or framework that can be tweaked for specific targets.

  .docb, .docm, .dot, .dotm, .dotx, .xlsm, .xlsb, .xlw, .xlt, .xlm, .xlc,
  .xltx, .xltm, .pptm, .pot, .pps, .ppsm, .ppsx, .ppam, .potx, .potm, .edb,
  .hwp, .602, .sxi, .sti, .sldx, .sldm, .sldm, .vdi, .vmdk, .vmx, .gpg, .aes,
  .ARC, .PAQ, .bz2, .tbk, .bak, .tar, .tgz, .gz, .7z, .rar, .zip, .backup,
  .iso, .vcd, .bmp, .png, .gif, .raw, .cgm, .tif, .tiff, .nef, .psd, .ai,
  .svg, .djvu, .m4u, .m3u, .mid, .wma, .flv, .3g2, .mkv, .3gp, .mp4, .mov,
  .avi, .asf, .mpeg, .vob, .mpg, .wmv, .fla, .swf, .wav, .mp3, .sh, .class,
  .jar, .java, .rb, .asp, .php, .jsp, .brd, .sch, .dch, .dip, .pl, .vb, .vbs,
  .ps1, .bat, .cmd, .js, .asm, .h, .pas, .cpp, .c, .cs, .suo, .sln, .ldf, .mdf,
  .ibd, .myi, .myd, .frm, .odb, .dbf, .db, .mdb, .accdb, .sql, .sqlitedb,
  .sqlite3, .asc, .lay6, .lay, .mml, .sxm, .otg, .odg, .uop, .std, .sxd,
  .otp, .odp, .wb2, .slk, .dif, .stc, .sxc, .ots, .ods, .3dm, .max, .3ds,
  .uot, .stw, .sxw, .ott, .odt, .pem, .p12, .csr, .crt, .key, .pfx, .der