| lproj | Language Project Folder |
| lps | Lazarus Project Session |
| lpz | Compiled SIMPL Windows Program |
| lref | WPF XAML Cache |
| lrs | Lazarus Resource |
| lsh | Lush Source Code |
| lsk | Liskell Source Code |
| lsl | Altium Designer Linker Script |
| lsp | Lisp Source Code |
| lss | GCC Assembler Listing File |
| lss | LotusScript Source Code |
| lst | GCC Assembler Listing File |
| lst | IAR System List Output Data |
| lst | µVision Assembler Listing Data |
| ltb | LithTech Binary File |
| lua | LUA Source Code |
| luac | LUA Compiled Source Code |
| lub | LUA Binary |
| luc | LuaTeX Compiled LUA Script |
| luca | Luca Template |
| luci | LuCI Script |
| lucidsnippet | Lucid Snippet Data |
| lvm | LabVIEW Measurement |
| lvr | Microsoft Test Manager Link |
| lxb | LXBatch Program |
| lxp | LXP Source Code |
| m | MATLAB Script |
| m1s | Mach3 Macro |
| m2o | Modula Fitted Modula2 Data |
| m2o | MSSQL-to-Oracle Profile Description Data |
| m3 | Modula-3 Source Code |
| m4 | M4 Macro File |
| m4m | SynDEx Makefile Macro |
| m4x | SynDEx Executive Macro |
| m51 | µVision Listing Data |