| itcl | Cygwin iwidgets Script |
| itf | JPI TopSpeed Pascal Interface File |
| itq | SAIA S-Web ITQ File |
| its | U-Boot uImage Image Tree Source |
| ivu | ImmerVision XML User Interface |
| ivy2 | Apache Ivy 2 Cache |
| iwb | IWBasic Source Code |
| iwdgt | iWeb Widget |
| iws | IntelliJ IDEA Workspace |
| ixx | Modula-3 Linker Information Data |
| izs | 1st Page 2000 Script |
| j | Java Source Code |
| j3o | JMonkey Engine 3D Object |
| jacl | Java Application Control Language Script |
| jadd | JastAdd Imperative Aspect |
| jade | Jade Markup Language Document |
| jak | AHEAD Tool Suite JAK File |
| jal | JAL Source Code |
| jam | Jam STAPL Source Code |
| jardesc | Eclipse JAR Description |
| jasper | JasperReports Compiled JRXML File |
| jav | Java Source Code |
| java | Java Source Code |
| javajet | Eclipse JET Java Template |
| jbc | Jam STAPL Byte-Code |
| jcc | JX C++ Source Code |
| jci | PSDsoft Express JTAG Chain Information |
| jck | Microsoft Liquid Motion JACK Scene Description |
| jcl | Job Control Language File |
| jcm | JECF Java Commerce Message |
| jco | CelLab Own Code |
| jcs | WebLogic Java Control Source |
| jcx | Java Control Extensions File |
| jdbc | JDBC Configuration |
| jdi | Altera Quartus II JTAG Debugging Information File |