Cuando IIS no tiene definida la extension que deseass ejecutar / descargar simplemente envia un error indicando que el archivo no existe… Bueno aunque..si existe .. pero como no reconoce la extension simplemente no te dejara hacer nada.

Para solucionar esto simplemente en el admonistrador de IIS seleccionas tu sitio web, Boton derecho/Propiedades y te vas a la pestaña de HTTP Headers.

Despues presionas el boton de MIME Map y anexas la extension con alguno de los tipos que esta en la lista de abajo.
Listo ya deberia de funcionar. Si tu MIME type no esta en la lista generalmente el proveedor de el tipo de archivo que quieres usar tendra el tipo correcto.

Todo por hoy…

Extension Tipo/sub-tipo IIS 4.0 IIS 5.0
* application/octet-stream Yes Yes
323 text/h323 No Yes
acx application/internet-property-stream No Yes
ai application/postscript Yes Yes
aif audio/x-aiff Yes Yes
aifc audio/x-aiff Yes Yes
aiff audio/x-aiff Yes Yes
asf video/x-ms-asf No Yes
asr video/x-ms-asf No Yes
asx video/x-ms-asf No Yes
au audio/basic Yes Yes
avi video/x-msvideo Yes Yes
axs application/olescript No Yes
bas text/plain Yes Yes
bcpio application/x-bcpio Yes Yes
bin application/octet-stream Yes Yes
bmp image/bmp Yes Yes
c text/plain Yes Yes
cat application/vnd.ms-pkiseccat No Yes
cdf application/x-cdf No Yes
cer application/x-x509-ca-cert No Yes
class application/octet-stream Yes Yes
clp application/x-msclip Yes Yes
cmx image/x-cmx Yes Yes
cod image/cis-cod Yes Yes
cpio application/x-cpio Yes Yes
crd application/x-mscardfile Yes Yes
crl application/pkix-crl No Yes
crt application/x-x509-ca-cert No Yes
csh application/x-csh Yes Yes
css text/css No Yes
dcr application/x-director Yes Yes
der application/x-x509-ca-cert No Yes
dir application/x-director Yes Yes
dll application/x-msdownload No Yes
dms application/octet-stream Yes Yes
doc application/msword Yes Yes
dot application/msword Yes Yes
dvi application/x-dvi Yes Yes
dxr application/x-director Yes Yes
eps application/postscript Yes Yes
etx text/x-setext Yes Yes
evy application/envoy Yes Yes
exe application/octet-stream Yes Yes
fif application/fractals No Yes
flr x-world/x-vrml Yes Yes
gif image/gif Yes Yes
gtar application/x-gtar Yes Yes
gz application/x-gzip No Yes
h text/plain Yes Yes
hdf application/x-hdf Yes Yes
hlp application/winhlp Yes Yes
hqx application/mac-binhex40 Yes Yes
hta application/hta No Yes
htc text/x-component No Yes
htm text/html Yes Yes
html text/html Yes Yes
htt text/webviewhtml No Yes
ico image/x-icon No Yes
ief image/ief Yes Yes
iii application/x-iphone No Yes
ins application/x-internet-signup No Yes
isp application/x-internet-signup No Yes
jfif image/pipeg No Yes
jpe image/jpeg Yes Yes
jpeg image/jpeg Yes Yes
jpg image/jpeg Yes Yes
js application/x-javascript Yes Yes
latex application/x-latex Yes Yes
lha application/octet-stream Yes Yes
lsf video/x-la-asf No Yes
lsx video/x-la-asf No Yes
lzh application/octet-stream Yes Yes
m13 application/x-msmediaview Yes Yes
m14 application/x-msmediaview Yes Yes
m3u audio/x-mpegurl No Yes
man application/x-troff-man Yes Yes
mdb application/x-msaccess Yes Yes
me application/x-troff-me Yes Yes
mht message/rfc822 No Yes
mhtml message/rfc822 No Yes
mid audio/mid No Yes
mny application/x-msmoney Yes Yes
mov video/quicktime Yes Yes
movie video/x-sgi-movie Yes Yes
mp2 video/mpeg Yes Yes
mp3 audio/mpeg No Yes
mpa video/mpeg Yes Yes
mpe video/mpeg Yes Yes
mpeg video/mpeg Yes Yes
mpg video/mpeg Yes Yes
mpp application/vnd.ms-project Yes Yes
mpv2 video/mpeg Yes Yes
ms application/x-troff-ms Yes Yes
mvb application/x-msmediaview Yes Yes
nws message/rfc822 No Yes
oda application/oda Yes Yes
p10 application/pkcs10 No Yes
p12 application/x-pkcs12 No Yes
p7b application/x-pkcs7-certificates No Yes
p7c application/x-pkcs7-mime No Yes
p7m application/x-pkcs7-mime No Yes
p7r application/x-pkcs7-certreqresp No Yes
p7s application/x-pkcs7-signature No Yes
pbm image/x-portable-bitmap Yes Yes
pdf application/pdf Yes Yes
pfx application/x-pkcs12 No Yes
pgm image/x-portable-graymap Yes Yes
pko application/ynd.ms-pkipko No Yes
pma application/x-perfmon Yes Yes
pmc application/x-perfmon Yes Yes
pml application/x-perfmon Yes Yes
pmr application/x-perfmon Yes Yes
pmw application/x-perfmon Yes Yes
pnm image/x-portable-anymap Yes Yes
pot, application/vnd.ms-powerpoint Yes Yes
ppm image/x-portable-pixmap Yes Yes
pps application/vnd.ms-powerpoint Yes Yes
ppt application/vnd.ms-powerpoint Yes Yes
prf application/pics-rules No Yes
ps application/postscript Yes Yes
pub application/x-mspublisher Yes Yes
qt video/quicktime Yes Yes
ra audio/x-pn-realaudio Yes Yes
ram audio/x-pn-realaudio Yes Yes
ras image/x-cmu-raster Yes Yes
rgb image/x-rgb Yes Yes
rmi audio/mid No Yes
roff application/x-troff Yes Yes
rtf application/rtf Yes Yes
rtx text/richtext Yes Yes
scd application/x-msschedule Yes Yes
sct text/scriptlet No Yes
setpay application/set-payment-initiation No Yes
setreg application/set-registration-initiation No Yes
sh application/x-sh Yes Yes
shar application/x-shar Yes Yes
sit application/x-stuffit No Yes
snd audio/basic Yes Yes
spc application/x-pkcs7-certificates No Yes
spl application/futuresplash No Yes
src application/x-wais-source Yes Yes
sst application/vnd.ms-pkicertstore No Yes
stl application/vnd.ms-pkistl No Yes
stm text/html Yes Yes
sv4cpio application/x-sv4cpio Yes Yes
sv4crc application/x-sv4crc Yes Yes
t application/x-troff Yes Yes
tar application/x-tar Yes Yes
tcl application/x-tcl Yes Yes
tex application/x-tex Yes Yes
texi application/x-texinfo Yes Yes
texinfo application/x-texinfo Yes Yes
tgz application/x-compressed No Yes
tif image/tiff Yes Yes
tiff image/tiff Yes Yes
tr application/x-troff Yes Yes
trm application/x-msterminal Yes Yes
tsv text/tab-separated-values Yes Yes
txt text/plain Yes Yes
uls text/iuls No Yes
ustar application/x-ustar Yes Yes
vcf text/x-vcard No Yes
vrml x-world/x-vrml Yes Yes
wav audio/x-wav Yes Yes
wcm application/vnd.ms-works Yes Yes
wdb application/vnd.ms-works Yes Yes
wks application/vnd.ms-works Yes Yes
wmf application/x-msmetafile Yes Yes
wps application/vnd.ms-works Yes Yes
wri application/x-mswrite Yes Yes
wrl x-world/x-vrml Yes Yes
wrz x-world/x-vrml Yes Yes
xaf x-world/x-vrml Yes Yes
xbm image/x-xbitmap Yes Yes
xla application/vnd.ms-excel Yes Yes
xlc application/vnd.ms-excel Yes Yes
xlm application/vnd.ms-excel Yes Yes
xls application/vnd.ms-excel Yes Yes
xlt application/vnd.ms-excel Yes Yes
xlw application/vnd.ms-excel Yes Yes
xof x-world/x-vrml Yes Yes
xpm image/x-xpixmap Yes Yes
xwd image/x-xwindowdump Yes Yes
z application/x-compress No Yes
zip application/zip Yes Yes