| hdb | Maple Help Database |
| hdc | HD Tach Benchmark Data |
| hde | HRS Pro Encrypted Secure File |
| hdf | Hierarchical Data Format File |
| hdf4 | Hierarchical Data Format File |
| hdf5 | Hierarchical Data Format Release 5 |
| hdg | HDGraph Vectorial Graph Data |
| hdh | FM-ONE Host Side Bootcontrol Data |
| hdi | HotDocs Auto-Install File |
| hdk | HotDocs Registration File |
| hdkn | Hadouken BitTorrent Client Data |
| hdl | HotDocs Template Library |
| hdo | HelpDesk One Data |
| hdo | Snapform Designer Field Layer Data |
| hdp | FM-ONE Parameters Data |
| hdpart | hyperDENT Parts Data |
| hdpartz | hyperDENT Parts Archive |
| hdpmx | HotDocs Publisher Mapping XML |
| hdpp | hyperDENT Global Allowance Data |
| hdr | Microsoft IIS NNTP Header Data |
| hdr | Mail.dat Header File |
| hdr | DataFlex Database Header |
| hdt | Snapform Designer Background Layer Data |
| hdu | FM-ONE User Data |
| hdumx | HotDocs User Mapping XML |
| hdz | Poser Compressed Hand Data |
| he2 | Hierarchical Data Format File |
| he4 | Hierarchical Data Format Release 4 Data |
| he5 | Hierarchical Data Format Release 5 |
| hea | PhysioBank Header File |
| hec | Pvquan Heckbert Data |
| hedata | HTML Executable Publication Data |
| hef | HEAT Export Data |
| heg | GRABLOX HEG Data |
| heinf | HippoEDIT File Information |