| .G4 | GTX RasterCAD |
| .GCA | GOCA graphics |
| .GED | Arts & Letters graphics |
| .GEM | GEM graphics |
| .GEO | VideoScope (filename extension). |
| .GIF | Graphics Interchange Format |
| .GLY | Glossary |
| .GP3 | CCITT compressed Group 3 TIFF file (filename extension). |
| .GP4 | Group 4 - CALS, ITU (filename extension). CCITT compressed Group 4 TIFF file. |
| .GRA | Microsoft Graph (filename extension). |
| .GRF | Graph |
| .GRP | Group |
| .GX1 | Show Partner graphics |
| .GX2 | Show Partner graphics |
| .GZ | GNU gzip compressed file - Unix |
| .H | Header (filename extension -- C). |
| .HDF | Hierarchical Data File |
| .HED | HighEdit document (filename extension). |
| .HEL | Microsoft Hellbender game file (filename extension). |
| .HGL | HP Graphics Language (filename extension). |
| .HLP | Help |
| .HPJ | Help Project - Visual Basic (filename extension). |
| .HPL | HP Graphics |
| .HQX | BinHex (filename extension -- Macintosh). |
| .HST | Host |
| .HT | HyperTerminal (filename extension). |
| .htm | DOS filename extension for a document with HTML coding, which can be read by WWW browsers. |
| .html | Filename extension for a document with HTML coding, which can be read by WWW browsers. |
| .HWP | Hangul Word Processor (filename extension). |
| .HYC | Hyphenation - WordPerfect |
| .HYP | Hyphenation |
| .ICA | IOCA graphics |
| .ICB | Targa bitmap (filename extension). |
| .ICM | Image Colour Matching (filename extension). |
| .ICO | Icon - Windows or OS/2 |
| .ICON | Sun icon and cursor |
| .IDD | Instrument Definition - MIDI (filename extension). |
| .IDE | Integrated Development Environment configuration (filename extension) |
| .IDX | Index |
| .IFF | Amiga |
| .IGF | Inset Systems graphics - Hijaak |
| .IIF | Interchange File - QuickBooks for Windows (filename extension). |
| .IL | Icon Library |
| .ima | Winimage compressed format These files can be decoded or mounted by ShrinkWrap. |
| .image | Mac disk image (filename extension); a compressed format which can be decoded or mounted by DiskCopy, Disk Image Mounter, Disk Charmer, and ShrinkWrap (ImageMaster). |
| .IMG | Image. This file is a Macintosh Disk Image. Disk Copy can be used to mount the disk image on the Macintosh desktop. The mounted disk image will behave like a read-only disk. |
| .IND | Index |
| .INF | Information |
| .INI | Initialization - configuration settings |
| .IOB | Imagine file (filename extension). |
| .ISU | Netscape file (filename extension). |
| .IT | MOD music file format (filename extension). |
| .IW | Idlewild screensaver (filename extension). |
| .IX | Index File - Framemaker (filename extension). |