| sec | PTC Creo Section Data |
| seco | MSX Configurator Software Update |
| secstore | Adobe Acrobat Security Store File |
| see | LISCAD Database |
| seg | Mail.dat Segment File |
| seg2 | SEG-2 Data File |
| segd | SEG-D Data File |
| segy | SEG Y Data File |
| sel | LinkOne Selection List |
| sel | SAM hmmscore Selection Data |
| sem3d | SEMulator3D Model Data |
| seman | Xtext Semantics Data |
| semi | Spartan Semi-Empirical Data |
| semul | WinZip Registration Data |
| senl | SAP Enable Now App Launcher |
| sep | SEEP/W Data |
| sep | SimEnhancer Profile |
| seq | CYANA Sequence File |
| seq | SAM sampleseqs Sequence Data |
| seq | Tinker Sequence |
| seq | Workswell Recorded Thermal Sequence |
| seq | XtalView Xfit Sequence Data |
| seqbac | SEQ Backup Data |
| server | Bonobo Activation API Data |
| ses | Adobe Audition v2 Session |
| ses | SPECCTRA Session File |
| sess | PHP Session Data |
| sesx | Adobe Audition Session |
| set | PLINK END-terminated Variant Set Membership List |
| sev | Sevilla Tournament Data |
| sevz | Sevilla Compressed Tournament Data |
| sew | Janome Stitch-based Embroidery Data |
| sez | SEEP/W ZIP Compressed Data |
| sf | RMCProfile Sulphur and Fluorine Data |
| sf | StatGraphics Centurion Data |