| .T | Holt Software | File sviluppatore | Turing Source Code File | Holt Software Turing | Holt Software |
| .TAL | Adobe Systems Incorporated | File sviluppatore | Adobe TypeAlign Text Illustration File | Adobe TypeAlign | Adobe Systems Incorporated |
| .TCL | Open Source | File sviluppatore | Tcl Script | MacroMates TextMate | MacroMates |
| .TDS | Borland | File sviluppatore | Turbo Debugger Symbols File | Borland Turbo Debugger | Borland |
| .TESTSETTINGS | Microsoft Corporation | File sviluppatore | Visual Studio Test Settings File | Microsoft Visual Studio | Microsoft Corporation |
| .TEXINFO | GNU Project | File sviluppatore | Texinfo Source Data | LaTex | Leslie Lamport |
| .TFP | Schneider Electric | File sviluppatore | ProWORX Nxt Full Page Data File | ProWORX | Schneider Electric |
| .TGML | Unknown Developer | File sviluppatore | Tecumseh Group Markup Language Source Code | Microsoft Notepad | Microsoft Corporation |
| .THML | Unknown Developer | File sviluppatore | Theological HTML Document | Theological Markup Language (ThML) | Unknown Developer |
| .THOR | Sublime HQ Pty Ltd | File sviluppatore | Thor Source Code | Notepad++ | Don Ho |
| .TI | timber-lang.org | File sviluppatore | Timber Compiler | Timber | timber-lang.org |
| .TIG | Texas Instruments, Inc. | File sviluppatore | TI Connect Backup File | Smith Micro StuffIt Deluxe | Smith Micro Software |
| .TIK | Unknown Developer | File sviluppatore | Script File For Game Textures | Unknown Software | Unknown Developer |
| .TK | Procedimientos-Uno, S.L. | File sviluppatore | TeKton3D Project | TeKton3D | Procedimientos-Uno, S.L. |
| .TKN | ShopTalk Systems | File sviluppatore | Liberty BASIC Application Distribution | Liberty Basic | ShopTalk Systems |
| .TKP | MicroImages | File sviluppatore | MicroImages SML Script Standalone Application | TNTmips | MicroImages |
| .TLD | The Eclipse Foundation | File sviluppatore | Tag Library Descriptor File | Notepad++ | Don Ho |
| .TLH | Microsoft Corporation | File sviluppatore | Typelib Generated C/C++ Header File | Microsoft Visual Studio | Microsoft Corporation |
| .TLI | Microsoft Corporation | File sviluppatore | Typelib Generated C/C++ Inline File | Microsoft C/C++ | Microsoft Corporation |
| .TML | Odetics Broadcast | File sviluppatore | AIRO Timeline File Format | AIRO | Odetics Broadcast |
| .TMPL | Xfire, Inc. | File sviluppatore | Xfire User Interface Template | Xfire | Xfire, Inc. |
| .TNS | Texas Instruments, Inc. | File sviluppatore | TI-Nspire Document | Texas Instruments TI-Nspire Software | Texas Instruments, Inc. |
| .TOKEN | Microspot | File sviluppatore | Sandcastle Help File Builder Token File | Microsoft Sandcastle | Microspot |
| .TOM | Aptora | File sviluppatore | Total Office Manager Accounting Data | Total Office Manager | Aptora |
| .TPR | Pegasys | File sviluppatore | TMPGEnc Project File | Media Player Classic | Open Source |
| .TPT | Cerious Software | File sviluppatore | ThumbsPlus Thumbnail Template | ThumbsPlus | Cerious Software |
| .TPU | Borland | File sviluppatore | Turbo Pascal Unit | Turbo Pascal | Embarcadero Technologies |
| .TPX | DeLorme | File sviluppatore | DeLorme Topo Project File | Topo North America | Interscape |
| .TQS | Parasoft | File sviluppatore | Insure++ Data | Insure++ | Parasoft |
| .TR2 | Yadabyte | File sviluppatore | TomeRaider eBook File | TomeRaider | Yadabyte |
| .TRC | Microsoft Corporation | File sviluppatore | SQL Server Trace File | Microsoft SQL Server | Microsoft Corporation |
| .TRT | Galactix Software | File sviluppatore | Team Sports Scheduling System Report Template | Team Sports Scheduling System | Galactix Software |
| .TRU | Original Basic | File sviluppatore | True Basic Source | True Basic Bronze Edition | Original Basic |
| .TRX | Microsoft Corporation | File sviluppatore | Visual Studio Test Results File | Microsoft Visual Studio | Microsoft Corporation |
| .TS | Various Developers | File sviluppatore | Video Transport Stream File | Squared 5 MPEG Streamclip | Open Source |
| .TSQ | Microsoft Corporation | File sviluppatore | ODBC Script File | Microsoft ODBC Data Source Administrator | Microsoft Corporation |
| .TT | Microsoft Corporation | File sviluppatore | Visual Studio Text Template | Microsoft Visual Studio | Microsoft Corporation |
| .TTI | Delphi Softwares | File sviluppatore | Trojan File | Beast (Trojan Horse) | Delphi Softwares |
| .TTR | Interscape | File sviluppatore | TextTransformer Productions | TextTransformer | Detlef Meyer-Eltz |
| .TU | Holt Software | File sviluppatore | Turing Source File | Holt Software Turing | Holt Software |
| .TUR | Holt Software | File sviluppatore | Turing Program Source File | Microsoft Notepad | Microsoft Corporation |
| .TXG | ESRI | File sviluppatore | Georeference Subobject File | ERDAS Imagine | Hexagon Geospatial |
| .TXL | Open Source | File sviluppatore | TXL Source Code | Notepad++ | Don Ho |
| .TXS | Delphi Softwares | File sviluppatore | Beast (Trojan Horse) Data File | Beast (Trojan Horse) | Delphi Softwares |
| .TXX | National Library of Medicine | File sviluppatore | Insight Segmentation And Registration Toolkit Data File | Insight Segmentation and Registration Toolkit | Open Source |
| .TYB | Tyler Johnson | File sviluppatore | TY-Basic Source Code File | Ty-Basic | Tyler Johnson |
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