| .T15 | H&R Block | Harvinaiset Tiedostot | H&R Block 2015 Tax Return | H&R Block Tax Software | H&R Block |
| .TCT | Adtollo | Harvinaiset Tiedostot | Topocad File | Topocad | Adtollo |
| .TD | Shenzhen Xunlei Networking Technologies | Harvinaiset Tiedostot | Thunder Incomplete Download File | Thunder | Shenzhen Xunlei Networking Technologies |
| .TEBK | Unknown Developer | Harvinaiset Tiedostot | Unknown File Type | Unknown Software | Unknown Developer |
| .TEC | SIL International | Harvinaiset Tiedostot | TECkit Compiled Mapping File | TECkit | SIL International |
| .TEMP | Safer Networking Ltd. | Harvinaiset Tiedostot | Temporary (Temp) File | FileAlyzer | Safer Networking Ltd. |
| .TFB | Unknown Developer | Harvinaiset Tiedostot | Unknown Apple II Data File | Unknown Software | Unknown Developer |
| .TG1 | Unknown Developer | Harvinaiset Tiedostot | On Target Project File | Unknown Software | Unknown Developer |
| .TGT | Corel | Harvinaiset Tiedostot | Corel Custom Data | CorelDRAW Graphics Suite | Corel |
| .THM | Sony | Harvinaiset Tiedostot | Sony Ericsson Mobile Phone Theme File | Sony Ericsson Theme Creator | Sony Mobile Communications |
| .THW | Unknown Developer | Harvinaiset Tiedostot | Unknown File Type | Unknown Software | Unknown Developer |
| .TIP | AVG Technologies | Harvinaiset Tiedostot | TuneUp Utilities Icon Package | AVG TuneUp - Unlimited | AVG Technologies |
| .TJF | Hewlett-Packard | Harvinaiset Tiedostot | VAXTPU Backup | OpenVMS | Hewlett-Packard |
| .TJT | Unknown Developer | Harvinaiset Tiedostot | Unknown File Type | Unknown Software | Unknown Developer |
| .TJZ | Unknown Developer | Harvinaiset Tiedostot | Unknown File Type | Unknown Software | Unknown Developer |
| .TKI | Unknown Developer | Harvinaiset Tiedostot | Mime: Application/x-tkined | Unknown Software | Unknown Developer |
| .TKO | Harman International Industries | Harvinaiset Tiedostot | AMX Software Module File | TPDesign4 | Harman International Industries |
| .TLB | Various Developers | Harvinaiset Tiedostot | VAX Text Library | Apple TextEdit | Apple |
| .TLS | AVG Technologies | Harvinaiset Tiedostot | TuneUp Utilities Logon Screen | AVG TuneUp - Unlimited | AVG Technologies |
| .TMB | ARRIS | Harvinaiset Tiedostot | Timbuktu Pro Connection Document | Timbuktu | ARRIS |
| .TML | Odetics Broadcast | Harvinaiset Tiedostot | AIRO Timeline File Format | AIRO | Odetics Broadcast |
| .TMO | Yu Higuchi | Harvinaiset Tiedostot | 3DCG Animation And Pose File | MikuMikuDance | Yu Higuchi |
| .TMPL | Xfire, Inc. | Harvinaiset Tiedostot | Xfire User Interface Template | Xfire | Xfire, Inc. |
| .TNEF | Microsoft Corporation | Harvinaiset Tiedostot | Transport Neutral Encapsulation Format | Microsoft Outlook Express | Microsoft Corporation |
| .TOAD | Unknown Developer | Harvinaiset Tiedostot | FunkyToad System File | FunkyToad Network | Unknown Developer |
| .TOC | Sony | Harvinaiset Tiedostot | Sony PlayStation Portable (PSP) Audio File | Exact Audio Copy for PSP Edition | Interscape |
| .TOO | DigiSoft Innovations | Harvinaiset Tiedostot | Asm Objasm File | Golden Orchard | DigiSoft Innovations |
| .TORRENT | BitTorrent | Harvinaiset Tiedostot | BitTorrent Information File | Shareaza | Open Source |
| .TR1 | IBM | Harvinaiset Tiedostot | IBM Voice Type Languages Addword File | IBM Voice Type | IBM |
| .TRACE | VMware | Harvinaiset Tiedostot | VMware ThinApp Trace Log File | VMware ThinApp 5 Suite | VMware |
| .TRANSFERIR | Unknown Developer | Harvinaiset Tiedostot | Unknown File Type | Unknown Software | Unknown Developer |
| .TRY | Microsoft Corporation | Harvinaiset Tiedostot | SCCM 2012 R2 Despoolr.box | Microsoft system center configuration manager | Microsoft Corporation |
| .TSA | Symantec Corporation | Harvinaiset Tiedostot | Jnana.tsa Malware | Avast Antivirus | Avast Software |
| .TSK | Microsoft Corporation | Harvinaiset Tiedostot | Pocket PC Skin | Microsoft ActiveSync | Microsoft Corporation |
| .TSTREAM | Open Source | Harvinaiset Tiedostot | SwarmPlayer Streaming Torrent | SwarmPlayer | Open Source |
| .TTI | Delphi Softwares | Harvinaiset Tiedostot | Trojan File | Beast (Trojan Horse) | Delphi Softwares |
| .TTO | Unknown Developer | Harvinaiset Tiedostot | Client Access Data Specification File | Unknown Software | Unknown Developer |
| .TTX | Simple DirectMedia Layer (SDL) | Harvinaiset Tiedostot | Trados TagEditor File | SDL Trados | SDL Trados |
| .TVLINK | Unknown Developer | Harvinaiset Tiedostot | Uncommon File Extension | Unknown Software | Unknown Developer |
| .TVT | IBM | Harvinaiset Tiedostot | IBM Lenovo ThinkVantage Rescue And Recovery Data File | ThinkVantage Rescue and Recovery | IBM |
| .TVW | Unknown Developer | Harvinaiset Tiedostot | Uncommon File Extension | Unknown Software | Unknown Developer |
| .TVX | Doug Cutting | Harvinaiset Tiedostot | Apache Lucene Term Vector Index File | Apache Lucene | Apache Software Foundation |
| .TVZ | Unknown Developer | Harvinaiset Tiedostot | Uncommon File Extension | Unknown Software | Unknown Developer |
| .TWK | FileMaker Inc. | Harvinaiset Tiedostot | T-Works Manager Data File | T-Works Manager | FileMaker Inc. |
| .TWR | Tripwire, Inc. | Harvinaiset Tiedostot | Tripwire Report File | Tripwire Enterprise | Tripwire, Inc. |
| .TX2 | Typequick Pty Ltd | Harvinaiset Tiedostot | Typequick File | Typequick | Typequick Pty Ltd |
| .TXG | ESRI | Harvinaiset Tiedostot | Georeference Subobject File | ERDAS Imagine | Hexagon Geospatial |
| .TXM | CFP Software Limited | Harvinaiset Tiedostot | CFPwinMan Document File | CFPwinMan | CFP Software Limited |
| .TXS | Delphi Softwares | Harvinaiset Tiedostot | Beast (Trojan Horse) Data File | Beast (Trojan Horse) | Delphi Softwares |
| .TX_ | Microsoft Corporation | Harvinaiset Tiedostot | Compressed TXT File | Microsoft Makecab | Microsoft Corporation |
| .TY2 | Unknown Developer | Harvinaiset Tiedostot | Gualtieri Theme Maker File | Gualtieri Theme Maker | Unknown Developer |
| .TYP | Unknown Developer | Harvinaiset Tiedostot | Thumbs32 File | Unknown Software | Unknown Developer |
| .TZD | AOL | Harvinaiset Tiedostot | Netscape Communicator Locale File | Netscape Communicator | NetScape |
| .TZG | Unknown Developer | Harvinaiset Tiedostot | Uncommon File Extension | Unknown Software | Unknown Developer |
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