Displays or modifies file extension associations

ASSOC [.ext[=[fileType]]]

  .ext      Specifies the file extension to associate the file type with
  fileType  Specifies the file type to associate with the file extension

Type ASSOC without parameters to display the current file associations.
If ASSOC is invoked with just a file extension, it displays the current
file association for that file extension.  Specify nothing for the file
type and the command will delete the association for the file extension.

c:\example>assoc .rtf=txtfile
Access is denied.
Error occurred while processing: .rtf.

c:\example>assoc .rtf=
Access is denied.

c:\example>assoc .rtf
.rtf=rtffile

c:\example>assoc
.386=vxdfile
.accountpicture-ms=accountpicturefile
.ani=anifile
.appcontent-ms=ApplicationContent
.application=Application.Manifest
.appref-ms=Application.Reference
.asa=aspfile
.asp=aspfile
.bat=batfile
.blg=Diagnostic.Perfmon.Document
.bmp=Paint.Picture
.cab=CABFolder
.camp=campfile
.cat=CATFile
.cdmp=cdmpfile
.cdx=aspfile
.cdxml=Microsoft.PowerShellCmdletDefinitionXML.1
.cer=CERFile
.chk=chkfile
.chm=chm.file
.cmd=cmdfile
.com=comfile
.compositefont=Windows.CompositeFont
.cpl=cplfile
.crl=CRLFile
.crt=CERFile
.css=CSSfile
.cur=curfile
.db=dbfile
.dctx=IMEDictionaryCompiler
.dctxc=IMEDictionaryCompiler
.dds=ddsfile
.der=CERFile
.desklink=CLSID\{9E56BE61-C50F-11CF-9A2C-00A0C90A90CE}
.deskthemepack=desktopthemepackfile
.diagcab=Diagnostic.Cabinet
.diagcfg=Diagnostic.Config
.diagpkg=Diagnostic.Document
.dib=Paint.Picture
.dll=dllfile
.docx=docxfile
.drv=drvfile
.dsn=MSDASQL
.emf=emffile
.eml=Microsoft Email Message
.evt=evtfile
.evtx=evtxfile
.exe=exefile
.fon=fonfile
.gif=giffile
.gmmp=gmmpfile
.grp=MSProgramGroup
.hlp=hlpfile
.hta=htafile
.htm=htmlfile
.html=htmlfile
.icc=icmfile
.icl=IconLibraryFile
.icm=icmfile
.ico=icofile
.imesx=imesxfile
.img=Windows.IsoFile
.inf=inffile
.ini=inifile
.iso=Windows.IsoFile
.jfif=pjpegfile
.Job=JobObject
.jod=Microsoft.Jet.OLEDB.4.0
.jpe=jpegfile
.jpeg=jpegfile
.jpg=jpegfile
.js=JSFile
.JSE=JSEFile
.jxr=wdpfile
.label=Label
.library-ms=LibraryFolder
.lnk=lnkfile
.log=txtfile
.mapimail=CLSID\{9E56BE60-C50F-11CF-9A2C-00A0C90A90CE}
.mht=mhtmlfile
.mhtml=mhtmlfile
.mlc=LpkSetup.1
.ms-windows-store-license=WindowsStore.License
.msc=MSCFile
.msi=Msi.Package
.msp=Msi.Patch
.msrcincident=RemoteAssistance.1
.msstyles=msstylesfile
.msu=Microsoft.System.Update.1
.mydocs=CLSID\{ECF03A32-103D-11d2-854D-006008059367}
.nfo=MSInfoFile
.ocx=ocxfile
.odt=odtfile
.osdx=opensearchdescription
.otf=otffile
.p10=P10File
.p12=PFXFile
.p7b=SPCFile
.p7c=certificate_wab_auto_file
.p7m=P7MFile
.p7r=P7RFile
.p7s=P7SFile
.partial=IE.AssocFile.PARTIAL
.pbk=pbkfile
.perfmoncfg=Diagnostic.Perfmon.Config
.pfm=pfmfile
.pfx=PFXFile
.pif=piffile
.pko=PKOFile
.pnf=pnffile
.png=pngfile
.ppkg=Microsoft.ProvTool.Provisioning.1
.prf=prffile
.printerExport=brmFile
.ps1=Microsoft.PowerShellScript.1
.ps1xml=Microsoft.PowerShellXMLData.1
.psc1=Microsoft.PowerShellConsole.1
.psd1=Microsoft.PowerShellData.1
.psm1=Microsoft.PowerShellModule.1
.pssc=Microsoft.PowerShellSessionConfiguration.1
.qds=SavedDsQuery
.rat=ratfile
.RDP=RDP.File
.reg=regfile
.resmoncfg=Diagnostic.Resmon.Config
.rle=rlefile
.rll=dllfile
.rtf=rtffile
.scf=SHCmdFile
.scp=txtfile
.scr=scrfile
.sct=scriptletfile
.search-ms=SearchFolder
.searchConnector-ms=SearchConnectorFolder
.settingcontent-ms=SettingContent
.sfcache=RDBFileProperties.1
.spc=SPCFile
.sst=CertificateStoreFile
.svg=svgfile
.symlink=.symlink
.sys=sysfile
.theme=themefile
.themepack=themepackfile
.tif=TIFImage.Document
.tiff=TIFImage.Document
.ttc=ttcfile
.ttf=ttffile
.txt=txtfile
.UDL=MSDASC
.URL=InternetShortcut
.VBE=VBEFile
.vbs=VBSFile
.vhd=Windows.VhdFile
.vhdpmem=Windows.VhdPmemFile
.vhdx=Windows.VhdFile
.vxd=vxdfile
.wab=wab_auto_file
.wbcat=wbcatfile
.wcx=wcxfile
.wdp=wdpfile
.webpnp=webpnpFile
.website=Microsoft.Website
.wmf=wmffile
.wsc=scriptletfile
.WSF=WSFFile
.WSH=WSHFile
.wtx=txtfile
.xaml=Windows.XamlDocument
.xbap=Windows.Xbap
.xht=xhtmlfile
.xhtml=xhtmlfile
.xml=xmlfile
.xrm-ms=MSSppLicenseFile
.xsl=xslfile
.ZFSendToTarget=CLSID\{888DCA60-FC0A-11CF-8F0F-00C04FD7D062}
.zip=CompressedFolder