| lpak | Lobster Program Archive |
| lpb | ProWorx NXT Logic Pointer with Blocking |
| lpf | ProWorx NXT Logic Data File |
| lpg | Sharp Ladder Logic Program File |
| lph | PC-lint Precompiled Header |
| lpi | Lazarus Project Information |
| lpr | Lazarus Program Source |
| 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 | 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 |