经常用IIS作为下载服务器的时候有时传上去的文件比如 example.mp4 文件名上传后，但是用http打开的时候确显示为 404 文件不存在。其实是IIS对文件的一种保护，不在IIS指定的MIME类型里的文件不会被操作。
常见的有 mp4 / flv / iso / 7z / apk 等扩展名的文件， iis 本身是没有指定MIME类型的，这类文件默认在IIS里是不能下载的，如确需下载，则需手动添加对应的 MIME 类型。
方法如下：
1、打开IIS，在右侧找到 MIME类型
比如我们需要IIS支持 MP4 文件下载可以这么设置：（这里我们对IIS全局进行设置、如果只针对某一个站点可以直接设置站点的）
2. 选中指定网站→在右侧找到 MIME类型 → 双击进入已有类型页 → 点击最右侧添加
3. 在弹出的 MIME类型框上 扩展名 MP4 MIME类型为： application/octet-stream
这样IIS就可以支持 MP4下载了。
下面列出一些常用的 扩展名的 MIME类型。
如果不知道MIME类型 可以写通用的: application/octet-stream
还有一些规律如果是文本类的让IE可以直接打开的 MIME 可以为 text/扩展名
如果是音频打开的时候让windows自动播放的可以用 audio/扩展名
再添几个常用的
apk application/vnd.android.package-archive
MRP文件（国内普遍的手机）
.mrp application/octet-stream
IPA文件(IPHONE)
.ipa application/iphone-package-archive
.deb application/x-debian-package-archive
APK文件(安卓系统)
.apk application/vnd.android.package-archive
CAB文件(Windows Mobile)
.cab application/vnd.cab-com-archive
XAP文件(Windows Phone 7)
.xap application/x-silverlight-app
SIS文件(symbian平台/S60V1)
.sis application/vnd.symbian.install-archive *（下有）
SISX文件(symbian平台/S60V3/V5)
.sisx application/vnd.symbian.epoc/x-sisx-app
扩展名类型/子类型
323 text/h323
acx application/internet-property-stream
ai application/postscript
aif audio/x-aiff
aifc audio/x-aiff
aiff audio/x-aiff
asf video/x-ms-asf
asr video/x-ms-asf
asx video/x-ms-asf
au audio/basic
avi video/x-msvideo
axs application/olescript
bas text/plain
bcpio application/x-bcpio
bin application/octet-stream
bmp image/bmp
c text/plain
cat application/vnd.ms-pkiseccat
cdf application/x-cdf
cer application/x-x509-ca-cert
class application/octet-stream
clp application/x-msclip
cmx image/x-cmx
cod image/cis-cod
cpio application/x-cpio
crd application/x-mscardfile
crl application/pkix-crl
crt application/x-x509-ca-cert
csh application/x-csh
css text/css
dcr application/x-director
der application/x-x509-ca-cert
dir application/x-director
dll application/x-msdownload
dms application/octet-stream
doc application/msword
dot application/msword
dvi application/x-dvi
dxr application/x-director
eps application/postscript
etx text/x-setext
evy application/envoy
exe application/octet-stream
fif application/fractals
flr x-world/x-vrml
gif image/gif
gtar application/x-gtar
gz application/x-gzip
h text/plain
hdf application/x-hdf
hlp application/winhlp
hqx application/mac-binhex40
hta application/hta
htc text/x-component
htm text/html
html text/html
htt text/webviewhtml
ico image/x-icon
ief image/ief
iii application/x-iphone
ins application/x-internet-signup
isp application/x-internet-signup
jfif image/pipeg
jpe image/jpeg
jpeg image/jpeg
jpg image/jpeg
js application/x-javascript
latex application/x-latex
lha application/octet-stream
lsf video/x-la-asf
lsx video/x-la-asf
lzh application/octet-stream
m13 application/x-msmediaview
m14 application/x-msmediaview
m3u audio/x-mpegurl
man application/x-troff-man
mdb application/x-msaccess
me application/x-troff-me
mht message/rfc822
mhtml message/rfc822
mid audio/mid
mny application/x-msmoney
mov video/Quicktime
movie video/x-sgi-movie
mp2 video/mpeg
mp3 audio/mpeg
mpa video/mpeg
mpe video/mpeg
mpeg video/mpeg
mpg video/mpeg
mpp application/vnd.ms-project
mpv2 video/mpeg
ms application/x-troff-ms
mvb application/x-msmediaview
nws message/rfc822
oda application/oda
p10 application/pkcs10
p12 application/x-pkcs12
p7b application/x-pkcs7-certificates
p7c application/x-pkcs7-mime
p7m application/x-pkcs7-mime
p7r application/x-pkcs7-certreqresp
p7s application/x-pkcs7-signature
pbm image/x-portable-bitmap
pdf application/pdf
pfx application/x-pkcs12
pgm image/x-portable-graymap
pko application/ynd.ms-pkipko
pma application/x-perfmon
pmc application/x-perfmon
pml application/x-perfmon
pmr application/x-perfmon
pmw application/x-perfmon
pnm image/x-portable-anymap
pot, application/vnd.ms-powerpoint
ppm image/x-portable-pixmap
pps application/vnd.ms-powerpoint
ppt application/vnd.ms-powerpoint
prf application/pics-rules
ps application/postscript
pub application/x-mspublisher
qt video/quicktime
ra audio/x-pn-realaudio
ram audio/x-pn-realaudio
ras image/x-cmu-raster
rgb image/x-rgb
rmi audio/mid
roff application/x-troff
rtf application/rtf
rtx text/richtext
scd application/x-msschedule
sct text/scriptlet
setpay application/set-payment-initiation
setreg application/set-registration-initiation
sh application/x-sh
shar application/x-shar
sit application/x-stuffit
snd audio/basic
spc application/x-pkcs7-certificates
spl application/futuresplash
src application/x-wais-source
sst application/vnd.ms-pkicertstore
stl application/vnd.ms-pkistl
stm text/html
svg image/svg+xml
sv4cpio application/x-sv4cpio
sv4crc application/x-sv4crc
swf application/x-shockwave-flash
t application/x-troff
tar application/x-tar
tcl application/x-tcl
tex application/x-tex
texi application/x-texinfo
texinfo application/x-texinfo
tgz application/x-compressed
tif image/tiff
tiff image/tiff
tr application/x-troff
trm application/x-msterminal
tsv text/tab-separated-values
txt text/plain