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