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