Just found this that supports the formats below:
http://www.smartftp.com/client/addons/sourcepreview
ActionScript (.as, .mx)
ASP (.asp)
ASPX (.aspx)
AutoIt (.au3)
Bash (.sh, .bsh)
Batch (.bat, .cmd)
Caml (.ml, .mli, .sml, .thy)
C (.c)
C++ (.cpp, .cxx, ,.cc, .h, .hxx, .rgs)
C# (.cs)
CSS (.css)
Diff (.diff, .patch)
Fortran (.f, .for, .f90, .f95, .f2k)
Ini (.ini, .inf, .reg, .url)
Inno Setup (.iss)
Java (.java)
JavaScript (.js)
Lisp (.lsp, .lisp)
Makefile (.mak)
Matlab (.m)
NFO (.nfo)
NSIS (.nsi, .nsh)
Pascal (.pas, .inc)
Perl (.pl, .pm, .plx)
PHP (.php, .php3, .phtml)
Python (.py, .pyw)
Resource (.rc)
Smalltalk (.st)
TeX (.tex)
SQL (.sql)
VB (.vb)
VBScript (.vbs)
XML (.xml, .xsd, .xsml, xsl, .kml)
See: https://stackoverflow.com/questions/9259574/how-to-preview-code-attachments-in-outlook/9265899
– PotatoFarmer – 2020-02-08T01:35:50.440