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