| I | Intermediate file (Borland C++) |
| I00 | WinShrink File (I-zero-zero. Usually followed with 1 or more .i0x files, and .md5 file) |
| IAN | Text file (Sterling Software) (Groundworks COOL Business Team Model) |
| IAX | Bitmap (IBM Image Acess eXecutive) |
| IBM | Compressed archive (ARCHDOS, IBM Internal only) |
| ICA | Bitmap graphic (Image Object Content Architecture) |
| ICB | Targa bitmap |
| ICC | Catalog file (IronClad) |
| ICD | SafeDisc encrypted program file |
| ICL | Icon Library (Generic industry standard) |
| ICM | Image Color Matching profile |
| ICN | Icon source code |
| ICO | Icon (Microsoft Windows 3.x) |
| ICS | Scene file (IronClad) |
| ID | Lotus Notes identification file |
| IDB | Intermediate file (Microsoft Developer) |
| IDD | MIDI Instrument Definition |
| IDE | Project file (Borland C++ v4.5) |
| IDF | MIDI Instrument Definition (Windows 95 required file) |
| IDIF | Identification file (Netscape saved address book) |
| IDL | OMG Interface Definition Language (IDL) file |
| IDQ | Cognos Impromptu Query Definition |
| IDW | Vector graphic (IntelliDraw) |
| IDX | Microsoft Outlook Express file |
| IFD | Adobe Output Designer forms definition source file |
| IFF | Image (Sun TAAC/SDSC Image Tool) |
| IFO | Graphic object layer data (ImageForge Pro) |
| IFP | Script file (KnowledgeMan) |
| IFS | System file (OS/2) |
| IGES | Initial Graphics Exchange Specification (Generic) |
| IGF | Metafile (Inset Systems) |
| IGs | Initial Graphics Exchange Specification (Generic) |
| IIF | Interchange file (QuickBooks for Windows) |
| IIM | Music module |
| ILB | Datafile (Scream Tracker) |
| ILBM | Bitmap (graphic image) |
| ILK | Program outline (Microsoft ILink incremental linker) |
| IM8 | Raster graphic (Sun Microsystems) |
| IMA | Vector graphic (EGO,Chart) |
| IMA | Image (WinImage) |
| IMF | MIDI music file (Corridor 7, Blake Stone, Wolfenstein 3D, Spear of Destiny) |
| IMG | Bitmap graphic (Ventura Publisher) |
| IMP | Spreadsheet (Lotus Improv) |
| IMQ | Image presentation (ImageQ) |
| IMR | Cognos Impromptu report |
| IMS | Create executable library data (IconForge) |
| IN$ | Installation file (HP NewWave) |
| IN3 | Input device driver (Harvard Graphics v3.0) |
| INB | Test script (Vermont High Test) |
| INC | Include file (Assembler language or Active Server) |
| IND | Shared Database file (Specifically in Microsoft Windows) |
| INDD | Adobe InDesign document |
| INF | Type I LaserJet font information file |
| INF | Information file (Generic) |
| INI | Setup file (MWave DSP synth's mwsynth.ini GM) |
| INK | Pantone reference fills file (CorelDRAW) |
| INL | Inline function file (Microsoft Visual C++) |
| INP | Source code for form, (Oracle,version 3.0 and earlier) |
| INRS | INRS-Telecommunications audio |
| INS | Install script (InstallShield) |
| INT | Interfaced units (Borland) |
| INTF | iNotes file |
| INTS | iNotes Secret file |
| INX | Index file (Foxbase) |
| IO | Compressed archive (CPIO) |
| IOB | 3D graphics database (TDDD format) |
| IOC | Organizational chart (Instant ORGcharting!) |
| IOF | Findit document (Microsoft Findit) |
| ION | File description (4dos descript.ion) |
| IPD | BlackBerry backup file |
| IPL | Pantone Spot reference pallette (CorelDRAW) |
| IPN | Autodesk Inventor presentation file |
| IPS | International patching system binary patch file |
| IPT | Autodesk Inventor part file |
| IQY | Internet inquiry (Microsoft) |
| IRS | optimized settings file (Adobe Illustrator and Photoshop) |
| ISD | Spell checker dictionary (RapidFile) |
| ISH | Compressed archive (ISH) |
| ISO | ISO (International Standards Organization table, aka: ISO) |
| IST | Instrument file (Digitaltracker) |
| ISU | Uninstall script (InstallShield) |
| IT | Music module (MOD) (Impulse Tracker) |
| IT | Settings file (intalk) |
| ITF | Interface file (JPI Pascal TopSpeed) |
| ITI | Instrument file (Impulse Tracker) |
| ITS | Internet document set (possibly a Microsoft file) |
| ITS | Sample file (Impulse Tracker) |
| IUD | Oratcl applications IUD Maker file |
| IV | Open Inventor file |
| IVD | Microdata dimension or variable-level file (Beyond 20/20) |
| IVP | User subset profile (Beyond 20/20) |
| IVT | Table or aggregate data (Beyond 20/20) |
| IVX | Microdata directory (Beyond 20/20) |
| IW | Presentation flowchart (IconAuthor-HSC Interactive) |
| IW | Screensaver (Idlewild) |
| IWA | Text file (IBM Writing Assistant) |
| IWC | Install Watch document |
| IWM | Start file (IconAuthor) |
| IWP | Text file (Wang) |
| IZT | Binary token file (IZT) |
